Explanation: A 3-circuit receiver typically refers to a regenerative receiver. The sensitivity of such a receiver, its ability to detect and amplify weak signals, is directly controlled by the **regeneration control**. By adjusting the regeneration, a portion of the amplified radio frequency (RF) signal is fed back to the input. This positive feedback increases the effective gain and selectivity of the RF amplifier stage. Setting the regeneration just below the point of oscillation maximizes the receiver's amplification, making it highly sensitive to weak radio signals. * **B) The setting of the audio frequency control** only adjusts the volume or tone of the *output audio* after the signal has been detected; it does not affect the receiver's ability to pick up weak RF signals. * **C) The setting of the radio frequency control** (e.g., tuning capacitor) selects the desired frequency but does not inherently control the *sensitivity* of the receiver in the way regeneration does. * **D) The setting of the beat frequency oscillator (BFO)** is used for demodulating Continuous Wave (CW) and Single Sideband (SSB) signals by creating an audible beat note. It does not control the general RF sensitivity of the receiver.