Question: Which Island is in the Town of Southold in Suffolk County, New York in the United States Plum Island or Block Island Sound?

Context paragraphs:
United States v. Maine: United States v. Maine, 469 U.S. 504 (1985), also known as the "Rhode Island and New York Boundary Case", was a decision by the U.S. Supreme Court, which held that the Long Island and Block Island Sounds in part constitute a juridical bay under Article 7(6), Long Island being an extension of the mainland and the southern headland of the bay, and (b) that the bay closed at the line drawn from Montauk Point at the eastern tip of Long Island to Watch Hill Point on the Rhode Island shore, the waters of the bay west of the closing line being internal state waters, and the waters of Block Island Sound east of that line being territorial waters and high seas.  Maine is named in the title of the case because it is the northernmost of the thirteen defendant states with coastline on the Atlantic Ocean in a series of cases related to overlapping claims of state and federal jurisdiction over seas and the seafloor.

Fort Terry: Fort Terry was a coastal fortification on Plum Island, a small island just off Orient Point, New York, USA.  This strategic position afforded it a commanding view over the Atlantic entrance to the commercially vital Long Island Sound.  It was established in 1897 and used intermittently through the end of World War II.  In 1952, it became a military animal and biological warfare (BW) research facility, moving to civilian control in 1954 as the Plum Island Animal Disease Center.  However, the biological warfare mission continued under civilian control until 1969, when the US ended offensive BW research.  The island is now being considered for sale or conversion to a wildlife refuge.

Little Gull Island: Little Gull Island is a small island in Long Island Sound, located approximately 0.4 mi northeast of Great Gull Island.  Both islands are located in the Town of Southold, in Suffolk County, New York, and lie roughly midway between Plum Island and Fishers Island.  Little Gull Island is approximately 4.6 mi southwest of Fishers Island and the channel of water between them is the main entrance to Long Island Sound, known as "The Race".

Long Island Sound: Long Island Sound is a tidal estuary of the Atlantic Ocean, lying between the eastern shores of Bronx County, New York City, southern Westchester County, and Connecticut to the north, and the North Shore of Long Island, to the south.  From east to west, the sound stretches 110 miles (177 km) from the East River in New York City, along the North Shore of Long Island, to Block Island Sound.  A mix of freshwater from tributaries and saltwater from the ocean, Long Island Sound is 21 miles (34 km) at its widest point and varies in depth from 65 to .

Plum Island (New York): Plum Island is an island in the Town of Southold in Suffolk County, New York in the United States.  The island is situated in Gardiners Bay, east of Orient Point, off the eastern end of the North Fork coast of Long Island.  It is about 3 mi long and 1 mi wide at its widest point.

Great Captain Island Light: Great Captain Island Lighthouse is a lighthouse on Great Captain Island in the western Long Island Sound off the coast of Greenwich, Connecticut, United States.  Built in 1829, the first lighthouse, made of stone, was of such poor construction that the walls were severely cracked a decade later.  In 1868, a new granite dwelling with attached lantern was completed.  The lighthouse is of the same design as lighthouses at Sheffield Island in Norwalk; Morgan Point in Noank; Old Field Point Light and Plum Island in New York; and Block Island North in Rhode Island.  In 1890, a fog whistle was added, in 1905 a siren was installed.  The lighthouse was deactivated in 1970 when a skeletal tower replaced it.  The Town of Greenwich acquired the property in 1973 and had full-time caretakers on the site until the lighthouse became too dilapidated in 2003.  A successful restoration effort was completed in 2009 and a non-navigational light was activated in 2012.  In 2010, a memorial plaque was installed to "honor the 23 people who lived in Greenwich, or had a connection to the town", who died in the September 11, 2001 attacks.  In 1991, the Great Captain Island Light was added to the National Register of Historic Places.

Block Island Sound: Block Island Sound is a strait in the open Atlantic Ocean, approximately 10 mi wide, separating Block Island from the coast of mainland Rhode Island in the United States.  On the west, it extends to Montauk Point on the eastern tip of Long Island, as well as Plum Island, Gardiners Island, and Fishers Island, all in the state of New York.

Great Gull Island: Great Gull Island is a 17 acre island separating Long Island and Block Island sounds, located approximately 0.4 mi southwest of Little Gull Island.  Both islands are located in Suffolk County, New York, and lie roughly midway between Plum Island and Fishers Island.

Block Island: Block Island is located off the coast of the U.S. state of Rhode Island, named after Dutch explorer Adriaen Block.  It is located in the Atlantic Ocean about 14 mi east of Montauk Point, Long Island, New York, and 13 mi south from mainland Rhode Island, from which it is separated by Block Island Sound.

USS Block Island (CVE-21): USS "Block Island" (CVE-21/AVG-21/ACV-21) was a "Bogue"-class escort carrier for the United States Navy during World War II.  She was the first of two escort carriers named after Block Island Sound off Rhode Island.  "Block Island" was launched on 6 June 1942 by Seattle-Tacoma Shipbuilding Corporation in Tacoma, Washington, under a Maritime Commission contract; sponsored by Mrs. H. B. Hutchinson, wife of Commander Hutchinson; transferred to the United States Navy on 1 May 1942; and commissioned on 8 March 1943, Captain Logan C. Ramsey in command.  Originally classified AVG-21, she became ACV-21 on 20 August 1942, and CVE-21 on 15 July 1943.  She was named after Block Island, an island in Rhode Island east of New York.

Supporting paragraphs: Block Island Sound Plum Island (New York)
Answer: Plum Island