Question

what is the purpose of a tuner in a radio?
the tuner converts a direct current into a high-frequency alternating current.
the tuner detects a wide range of broadcast frequencies.
the tuner selects only the signal of a specific frequency and rejects the rest.
the tuner converts inaudible high-frequency signals into sound waves that can be heard.

Explanation:

A radio tuner's core function is to isolate the specific radio frequency of the desired station from the many overlapping broadcast frequencies available, ignoring all others. The other options describe functions of other radio components: converting DC to high-frequency AC is an oscillator's role, detecting a wide range of frequencies is not its purpose (it targets one), and converting high-frequency signals to sound is done by speakers/amplifiers after demodulation.

Answer:

The tuner selects only the signal of a specific frequency and rejects the rest.