‘Tor’ is

Concept: In physical geography, landforms produced by differential (selective) weathering are classified by their scale and origin. When chemical and physical weathering attacks rock along joints and fractures faster than it attacks the surrounding mass, a core of more resistant, unweathered rock survives while the decomposed material (regolith) around it is stripped away by erosion.

Application: This surviving core of resistant bedrock is left standing proud of the surface as a small, rounded, rocky hill or outcrop, typically on a hillside or hilltop — this exposed residual rock mass is exactly what geographers term a "Tor." So "Tor" refers to small hills of exposed bedrock formed by differential weathering.

Cross-check: Compare each remaining option by what it actually denotes, to confirm none of them fits the term "Tor":

• Peneplain: a nearly flat, low-relief surface produced when a landscape is worn down by erosion over a very long geological period until it approaches base level — the opposite of a small residual hill.

• Valley: an elongated lowland or depression lying between hills or mountains, typically excavated by a river or glacier — a negative (cut-into) landform, not a standing rock mass.

• Plateau: an extensive, elevated landform with a broadly flat top, standing above the surrounding land over a wide area — much larger in scale than a single rocky hillock.

Result: None of Peneplain, Valley, or Plateau describes an isolated residual rock outcrop, so "Tor" is correctly defined as small hills.