In an SSB/CW receiver, what is the purpose of the intermediate frequency (IF) amplifier?

In a receiver, 'gain' refers to the amplification of the signal. While the initial radio frequency (RF) amplifier boosts the tiny antenna signal slightly to overcome mixer noise, the vast majority of the signal amplification happens in the intermediate frequency (IF) amplifier. Because the IF amplifier operates at a single, fixed frequency, it can be designed to provide enormous amounts of stable, high-gain amplification without breaking into unwanted oscillation. Trying to achieve this much gain at a variable, high-frequency RF stage would be extremely difficult. The other options describe different stages: boosting the recovered audio happens in the audio frequency (AF) amplifier, and providing initial boost for the mixer is the job of the RF amplifier.