Question: Valhalla is a hamlet and census-designated place (CDP) located within the town of Mount Pleasant, in Westchester County, is a county in the U.S. state of New York, it is the second-most populous county on the mainland of New York, after which location?

Context paragraphs:
Scotts Corners, New York: Scotts Corners is a hamlet (and census-designated place or CDP) located in the town of Pound Ridge in Westchester County, New York, United States.  As of the 2010 census, the CDP population was 711, making Scotts Corners Westchester County's smallest census-recognized area.

Thornwood, New York: Thornwood is a hamlet and census-designated place (CDP) in the town of Mount Pleasant in Westchester County, New York.  The population was 3,759 at the 2010 census.

Mount Pleasant, New York: Mount Pleasant is a town in Westchester County, New York, in the New York metropolitan area.  As of the 2010 census, the town population was 43,724.  The villages of Valhalla, Hawthorne, Thornwood, Pleasantville, Sleepy Hollow, and a small portion of Briarcliff Manor lie within the town, as well as a number of hamlets.

Hawthorne, New York: Hawthorne is a hamlet and census-designated place (CDP) located in the town of Mount Pleasant in Westchester County, New York.  The population was 4,586 at the 2010 census.

Westchester County, New York: Westchester County is a county in the U.S. state of New York.  It is the second-most populous county on the mainland of New York, after the Bronx.  According to the 2010 Census, the county had a population of 949,113, estimated to have increased by 2.7% to 974,542 by 2016.  Situated in the Hudson Valley, Westchester covers an area of 450 sqmi , consisting of 6 cities, 19 towns, and 23 villages.  Established in 1683, Westchester was named after the city of Chester, England.  The county seat is the city of White Plains, while the most populous municipality in the county is the city of Yonkers, with an estimated 200,807 residents in 2016.

Briarcliff Manor, New York: Briarcliff Manor is a suburban village in Westchester County, New York, around 30 mi north of New York City.  It is on 5.9 sqmi of land on the east bank of the Hudson River, geographically shared by the towns of Mount Pleasant and Ossining.  Briarcliff Manor includes the communities of Scarborough and Chilmark, and is served by the Scarborough station of the Metro-North Railroad's Hudson Line.  A section of the village, including buildings and homes covering 376 acre , is part of the Scarborough Historic District and was listed on the National Register of Historic Places in 1984.  The village motto is "A Village between Two Rivers", reflecting Briarcliff Manor's location between the Hudson and Pocantico Rivers. Although the Pocantico is the primary boundary between Mount Pleasant and Ossining, since its incorporation the village has spread into Mount Pleasant.

Sleepy Hollow, New York: Sleepy Hollow is a village in the town of Mount Pleasant in Westchester County, New York.  The village is located on the east bank of the Hudson River, about 30 mi north of New York City, and is served by the Philipse Manor stop on the Metro-North Hudson Line.  To the south of Sleepy Hollow is the village of Tarrytown, and to the north and east are unincorporated parts of Mount Pleasant.  The population of the village at the 2010 census was 9,870.

Middlesex County, New Jersey: Middlesex County is a county located in north-central New Jersey, United States.  As of the 2016 Census estimate, the county's population was 837,073, making it the state's second-most populous county, an increase of 3.4% from the 2010 United States Census, when its population was enumerated at 809,858, making it the second-most populous county in the state.  Middlesex is part of the New York metropolitan area, and its county seat is New Brunswick.  The center of population of the state of New Jersey is located in Middlesex County, in East Brunswick Township, just east of the New Jersey Turnpike.  The 2000 Census showed that the county ranked 63rd in the United States among the highest-income counties by median household.  The Bureau of Economic Analysis ranked the county as having the 143rd-highest per capita income of all 3,113 counties in the United States (and the 10th-highest in New Jersey) as of 2009.  Middlesex County holds the nickname, "The Greatest County in the Land".

New York State Route 448: New York State Route 448 (NY 448) is a 3.90 mi long state highway in western Westchester County, New York, in the United States.  The route begins in the village of Sleepy Hollow at U.S. Route 9 (US 9) and goes in a northeast direction through the Pocantico Hills community in Mount Pleasant.  It ends at NY 117 in Mount Pleasant, near the junction of NY 117 with Saw Mill River Road (NY 9A and NY 100) and the Taconic State Parkway.  From 1930 to November 1970, NY 448 was part of NY 117.

Valhalla, New York: Valhalla is a hamlet and census-designated place (CDP) located within the town of Mount Pleasant, in Westchester County, New York, United States, in the New York City metropolitan area.  Its population was 3,162 at the 2010 U.S. Census.  The name of the community was inspired by a fan of the opera composer Richard Wagner, and the hamlet is known both for its location as the home of the primary hospital campus of Westchester Medical Center and New York Medical College, as well as burial places of the remains of numerous well-known people.  The name comes from "Valhalla", a heavenly abode in Norse mythology.

Supporting paragraphs: Valhalla, New York Westchester County, New York
Answer: Bronx