Question: Robert the Bruce extended a castle located on the southern shore of which Loch ?

Context paragraphs:
Loch Dunvegan: Loch Dunvegan (Scottish Gaelic: "Loch Dhùn Bheagain"), is a sea loch on the west coast of the island of Skye in the Inner Hebrides of Scotland.  Dunvegan, the village it was named after is located by its southern shore.  The settlements of Galtrigill, Borreraig, Uig and Colbost are located on its western shore.  Claigan is located on its eastern shore.

West Loch Tarbert: West Loch Tarbert (Scottish Gaelic: "Loch A Siar" ) is a sea loch that separates the northern and southern parts of the island of Harris in the Outer Hebrides of Scotland.  A small isthmus joins these two areas, on which is to be found the village of Tarbert.  The loch contains the islands Soay Mòr, Soay Beag and Isay, while Taransay lies near the mouth of the loch.  Amhuinnsuidhe Castle is located on the northern shore of the loch.

Tarbert Castle: Tarbert Castle is located on the southern shore of East Loch Tarbert, at Tarbert, Argyll, Scotland, at the north end of Kintyre.  Tarbert Castle was a strategic royal stronghold during the Middle Ages and one of three castles at Tarbert.  The castle overlooks the harbour and although pre 14th century in construction, the tower dates back to 1494 and the visit of James IV to the Western Highlands.

Castle Semple Loch: Castle Semple Loch is a 1.5-mile-long (2.5 km) inland freshwater loch at Lochwinnoch in Renfrewshire, Scotland.  Originally part of an estate of the same name, it is now administered by Clyde Muirshiel Regional Park as a watersports centre.  An RSPB bird sanctuary is located on the loch's southern shore.

Tarbertshire: Tarbertshire, or the sheriffdom of Tarbert, was a shire of Scotland until 1633, when it was annexed to Argyllshire.  It comprised the mainland pensinsula formed by Knapdale and Kintyre, together with the southern Inner Hebrides to the west.  It has been suggested that Robert the Bruce, who extended Tarbert Castle, created or intended a sheriffdom there; however, the first reference to Tarbertshire is in 1481, when Knapdale (back then including the lands between Loch Awe and Loch Fyne) was transferred to it from Perthshire.  The shire town was Tarbert, but the sheriff court was latterly at Inveraray in Argyllshire.  The Earl of Argyll family of Campbells often supplied the Sheriff of Tarbert and other officials of both Argyll and Tarbert.

Loch Sween: Loch Sween is a sea loch located near Lochgilphead, Argyll and Bute, Scotland.  Castle Sween is located on the southern shore towards the seaward end of the loch.  The village of Tayvallich, a favoured haven for water craft as it sits at the head of sheltered Loch a' Bhealaich, lies on the northern shore.

Lochend Castle, Campbeltown: Lochend Castle (Scottish Gaelic: "Caislen Cean Loch" , Anglicised "Castle of the loch end"), was a castle located at Campbeltown, Argyll and Bute.  The castle once stood at the top of Castlehill, Campbeltown and was once occupied by Clan MacDonald of Dunnyveg.

Loch Arienas: Loch Arienas is a large, lowland, freshwater loch on the Ardtornish Estate on the Morvern peninsula in the Scottish Highlands.  It lies in a west-northwest to east-southeast direction, is approximately 3.2 km long and 1.21 km wide, and is at an altitude of 12 m .  The northern shore of the loch is mostly regular in shape, while the southern shore is irregular.  Its average depth is 17.2 m and its maximum depth is 35 m .  The loch was surveyed on the 18 and 19 of August 1904 by John Hewitt as part of Sir John Murray's "Bathymetrical Survey of Fresh-Water Lochs of Scotland 1897-1909".

Rosneath: Rosneath ("Ros Neimhidh" in Gaelic) is a village in Argyll and Bute, Scotland.  It sits on the western shore of the Gare Loch near to the tip of the Rosneath peninsula which projects south to the Firth of Clyde between the Gare Loch and Loch Long to the west , and about 2 mi from the village of Kilcreggan which is sited on the southern end of the peninsula, on the shores of the Firth of Clyde.  The large town of Greenock is directly opposite on the southern shore of the Firth.  The Gare Loch narrows at Rosneath to under half a mile (around 600 metres) at a place known as the "Rhu Narrows" after the village of Rhu on the eastern shore of the loch.

Loch Avich: Loch of Avich is a large freshwater loch that lies approximately 1 mi west of Loch Awe in Argyll and Bute in Scotland.  It is the second biggest loch in the Etive basin after Loch Awe.  It trends east-northeast, west-southwest and is narrowly triangular in shape.  At the southwest end of the loch is the islet "Innis Luana" on which stands the ruins of an ancient castle named "Caisteal na Nighinn Ruaidhe" (Castle of the Red Haired Maiden).  The castle may have been the original seat of Clan Campbell.  Near the north shore on the islet of "Eilean Fraoch" is the remains of a crannog.

Supporting paragraphs: Tarbertshire Tarbert Castle
Answer: East Loch Tarbert