Explanation: A constant-k filter is a basic type of image parameter filter. A key characteristic of its design is that its attenuation, while not always having the steepest roll-off immediately past the cutoff frequency, *continually increases* as the frequency moves further into the stopband. This provides very high attenuation for signals at frequencies significantly removed from the passband, effectively blocking distant unwanted signals. Here's why the other options are incorrect: * **B) It can match impedances over a wide range of frequencies.** Constant-k filters have a characteristic impedance that is only constant in the passband and varies significantly in the stopband, making them unsuitable for wideband impedance matching. * **C) It uses elliptic functions.** Elliptic (Cauer) filters are a more complex design that use elliptic functions to achieve a very steep roll-off and equiripple in both the passband and stopband. Constant-k filters are much simpler. * **D) The ratio of the cutoff frequency to the trap frequency can be varied.** Filters with a "trap frequency" (a frequency of infinite attenuation) are typically m-derived filters (a variation of constant-k filters) or elliptic filters, not the basic constant-k filter itself.