Explanation: Receiver hiss is primarily due to thermal noise, and its impact is most significant when introduced early in the signal chain. Noise generated in the first detector (often a mixer or a simple detector stage following an RF amplifier) is amplified by all subsequent stages. This is critical because the noise figure of a receiver is dominated by the noise characteristics of its initial stages. Any noise added at these early points directly degrades the signal-to-noise ratio (SNR) that the rest of the receiver processes. Later stages, like the first AF amplifier, introduce their own noise, but this noise is added to an already amplified signal and the noise from previous stages, thus having a proportionally smaller impact on the overall SNR. Faulty regeneration control (C) would typically cause distortion or oscillation, not fundamental receiver hiss. Therefore, noise from the first detector is the primary culprit for receiver hiss.