The correct answer is (D) To assume or take shape; to become clear, substantial, or definite; to congeal, to become jellylike; to assume the consistency of a jelly.

Jell comes from jelly (English), which derives from Latin gelare (to freeze).

Examples of use:

• As the meeting progressed, their plans jelled.
• She stirred the pudding for several minutes before it began to jell.