Explanation: RADAR receivers typically utilize a superheterodyne architecture, which involves downconverting the high-frequency radio frequency (RF) signal to a lower intermediate frequency (IF) for easier amplification and processing. The component responsible for this initial frequency conversion is the mixer. In many microwave applications, including RADAR, a **crystal detector** (often a diode like a point-contact diode or Schottky diode) is used as the mixer. Its non-linear characteristic is crucial for combining the incoming RF signal with a local oscillator (LO) signal to produce the IF signal. Therefore, block 49, representing this essential first frequency conversion stage in the receiver, is correctly identified as the crystal detector, functioning as the mixer. A discriminator (A) is used for FM demodulation. An IF amplifier (B) amplifies the signal *after* the mixer. A klystron (C) is typically a high-power microwave amplifier or oscillator used in the transmitter section or as an LO source, not the mixer itself.