The primary job of a(n) on a receiver is to capture modified radio waves.

The correct answer is an antenna.

Explanation:

In radio communication, radio receives radio waves which is an electromagnetic wave. It converts the information carried by them to usable form.

An antenna intercepts radio waves. It converts  them to tiny alternating current which are applied to receiver. Then, receiver extracts the desired information.

Therefore, the primary job of an antenna on a receiver is to capture modified radio waves