Explanation: In a pulse-modulated RADAR system, the received signal at the "video (second) detector" stage consists of amplitude-modulated (AM) pulses on an intermediate frequency (IF) carrier. The detector's role is to extract these baseband pulses from the carrier. A **diode detector (B)** is an envelope detector specifically designed for amplitude-modulated signals. It rectifies the incoming IF pulses, effectively stripping away the high-frequency carrier and leaving the modulating envelope—the original video pulses. This process is simple, efficient, and perfectly suited for recovering the pulse information used for target detection and display. **Discriminator (A)** and **ratio detectors (C)** are used for frequency modulation (FM) detection, where information is encoded in frequency variations, not amplitude. Since pulse RADAR primarily uses amplitude modulation, these detectors are unsuitable. An **infinite impedance detector (D)** is a type of AM envelope detector, typically tube-based and more complex than a simple diode, offering no significant advantage over a diode for fast-response pulse detection in most modern RADAR systems.