The prefix semi-,as in the term semilunar,means:

A) above.
B) under.
C) large.
D) half.

Semi-

A prefix meaning half or partially, often used to indicate that something is incomplete or limited.

Semilunar

Shaped like a half-moon or crescent; often used to describe certain valves or cartilages in the human body that have a crescent shape.

Half

Refers to one of two equal parts into which something is or can be divided.