In the following question, out of the given four alternatives, select the one which is opposite in meaning of the given word.
Hummock

Hummock refers to a small hill, knoll, or mound of earth/ice (an elevated piece of land).
Ridge, Hill, and Upland are all related to elevated landforms (synonymous in nature).

Ditch means a narrow channel dug in the ground, typically for drainage (a depression or low ground), which is the exact opposite of an elevated mound.

Therefore, the correct option is D.