Which of the following is a component of a RADAR unit?

A power source is indeed a critical component of a RADAR unit as it provides the necessary energy to operate the system. In a RADAR unit, the power source energizes the transmitter, which emits radio waves. The functionality of RADAR relies on a continuous supply of power to both transmit signals and process received echoes.

While a mirror, sensor, and display screen may be relevant in certain contexts (such as in specific types of optical systems or interface components), they are not fundamental to the core operation of a RADAR system. The power source is essential for the entire operation of the RADAR, making it indispensable for the system to function effectively.