Thick sloping or battered walls, grey stone and very plain surfaces are the mark of which dynasty of the Delhi Sultanate?

The correct answer is C, Tughlaq dynasty. Tughlaq building is the most austere phase of the Sultanate style. It uses local grey sandstone or rubble, keeps ornament to a minimum and gives its walls a pronounced batter, that is a slope inward as they rise, which makes even a tomb look like a fort. Tughlaqabad built by Ghiyas-ud-din Tughlaq, his tomb joined to the fort by a causeway, Firoz Shah Kotla and the Hauz Khas complex all show this manner.

Option A, the Slave dynasty, worked in red sandstone with rich carving taken from temple material, as at the Qutub complex. Option B, the Khalji dynasty, brought the true arch and the marble inlay of the Alai Darwaza, which is decorative rather than plain. Option D, the Lodi dynasty, is known for the double dome and for garden tombs on octagonal and square plans, with glazed tile work. The sloping wall belongs to the Tughlaqs.