Explanation: In a superheterodyne receiver, the incoming radio frequency (RF) signal is mixed with a local oscillator (LO) signal to produce a fixed Intermediate Frequency (IF) signal. This IF signal is then amplified by the IF amplifier stages, which are always tuned to this specific IF frequency, regardless of the original RF frequency. The second detector's purpose is to extract the intelligence (audio, data, etc.) from this amplified IF signal. To do this efficiently, its input circuit (the grid circuit in a tube-based receiver, or equivalent input in a solid-state design) must be tuned to the IF frequency. Therefore, it will always be tuned to 475 kHz. The other options represent either the original RF signal, possible local oscillator frequencies, or harmonics, none of which are directly applied to the second detector for demodulation.