Which of the following filter types is not suitable for use at audio and low radio frequencies?

A cavity filter operates using physical resonance, meaning the physical dimensions of the metallic cavity must be a significant fraction of the wavelength of the signal it is filtering. This makes them highly efficient at VHF, UHF, and microwave frequencies where wavelengths are short. At audio and low radio frequencies, however, wavelengths are incredibly long—often spanning hundreds to millions of meters. A resonant cavity filter designed for these low frequencies would need to be the size of a large building or bigger, making it physically impossible for practical use. Instead, standard lumped-element filters like Chebyshev, Butterworth, and Elliptical designs are easily constructed for low frequencies using appropriately sized capacitors and inductors.