Explanation: An MTI (Moving Target Indicator) circuit in a radar receiver is designed to filter out stationary echoes, known as clutter, by utilizing the Doppler effect. Moving targets cause a frequency (or phase) shift in the reflected radar signal, while stationary objects do not. The MTI circuit compares successive echoes and only processes signals that show this phase shift. This effectively removes returns from fixed objects like terrain, buildings, or weather, allowing only targets that are in motion relative to the radar to be displayed. Therefore, the primary function of an MTI circuit is to ensure that only moving targets are visible on the display. Options A and C are incorrect because MTI is not related to receiver gain control or the disabling of AGC circuits; its function is target discrimination. Option D is incorrect as "rabbits" typically refer to spurious echoes or interference, and MTI's purpose is to *remove* ground clutter itself, not to make clutter free of "rabbits."