Explanation: Selectivity in a receiver is the ability to distinguish between a desired signal and unwanted signals on nearby frequencies. In the front-end circuitry, this is crucial to prevent strong out-of-band signals from overloading the receiver or creating image frequencies and intermodulation products. A **preselector** (D) is a tunable bandpass filter located at the very front end of a receiver, typically before the RF amplifier and mixer. Its purpose is to pass only the desired range of RF frequencies while rejecting other signals, thus providing crucial initial selectivity for the front end. Options A, B, and C are incorrect because: * **A) An audio filter** operates on the signal after it has been demodulated to audio frequencies, long after the RF front end. It affects audio quality but not RF selectivity. * **B) An additional RF amplifier stage** primarily increases the strength of the incoming RF signal (gain), not its selectivity, unless it's part of a tuned circuit specifically designed for filtering. * **C) An additional IF amplifier stage** operates on the Intermediate Frequency signal, which is generated *after* the RF signal has passed through the front-end mixer. While IF filters provide the receiver's main selectivity, an IF stage is not part of the *front-end RF circuitry*.