What is the purpose of the notch filter found on many HF transceivers?

A notch filter is an extremely narrow band-stop filter designed to attenuate a very specific, narrow range of frequencies while allowing the rest of the audio or radio frequency spectrum to pass through unaffected. On an HF transceiver, its primary job is to target and 'notch out' specific continuous tones. During HF reception, continuous interfering carriers—such as someone tuning up their transmitter, a continuous wave (CW) signal, or an AM broadcast carrier—create loud, annoying heterodynes or whistles in the receiver passband. By adjusting the notch filter to the exact frequency of that tone, the operator can eliminate the squeal while preserving the intelligibility of the desired voice signal. The incorrect options describe different types of receiver features. Removing impulse noise (like ignition static) is the function of a Noise Blanker. Removing wideband adjacent signal splatter generally requires narrowing the receiver's overall IF bandwidth using bandpass filters, since a notch filter is too narrow to handle widespread interference.