The correct answer is (D) To reduce, alleviate, diminish, lessen, relieve, make less (anger, doubt, fear, pain, suspicion, etc.); to repress, put down, put to rest, calm, quiet.

Allay comes from Middle English aleyen, Old English alecgan (to put down).

Examples of use:

• As promised, the pill soon allayed his pain.
• Her explanation allayed their doubts.