Explanation: A mixer circuit, also known as a first detector in a superheterodyne receiver, operates by combining two input signals—a radio frequency (RF) signal and a local oscillator (LO) signal—to produce sum and difference frequencies. This process is called heterodyning. In a RADAR system, the mixer's purpose is to combine the received echo (RF signal) with the local oscillator frequency to generate an intermediate frequency (IF) that can be easily amplified and processed. If no echoes are being received, there is no RF input signal for the mixer to combine with the local oscillator signal. Without both necessary inputs, the mixer cannot perform its frequency conversion function. Therefore, no intermediate frequency output is developed. Options A, B, and C represent frequencies that would be present if mixing occurred, or are just the LO frequency, not the *output* of the mixing process itself when no echo is present.