A strong interfering signal is very close to your receive frequency. Which type of filter can effectively suppress it?

When an interfering signal is very close to your desired receive frequency, you need a filter that can surgically remove the offending signal without severely affecting the audio or signals immediately adjacent to it. A notch filter is specifically designed to provide high attenuation (a deep 'notch') over a very narrow range of frequencies. It allows the frequencies both immediately above and below the notch to pass through relatively unaffected. Low-pass and high-pass filters would cut off a massive portion of the spectrum, likely taking your desired signal with it. A standard band-reject filter operates similarly to a notch filter but typically affects a much wider swath of frequencies, which would also swallow up your desired nearby signal.