In a superheterodyne receiver, which stage is called the front-end?

The 'front-end' of a superheterodyne receiver is the very first stage that intercepts the incoming radio signal directly from the antenna. In most receiver architectures, this first stage is the Radio Frequency (RF) amplifier, which is responsible for boosting the extremely weak signals captured by the antenna before they are passed further down the chain. Other options play roles much later in the signal path. The Local Oscillator generates a signal used for mixing, but it does not process the incoming antenna signal directly. The Limiter and Audio Frequency (AF) amplifier process the signal after it has already been converted and demodulated. Because the RF amplifier is the gateway for all incoming signals, its performance heavily dictates the overall sensitivity and quality of the receiver, making 'front-end' a fitting industry nickname.