What is used to measure frequency in an AC circuit?

In an AC circuit, frequency refers to the number of cycles per second, measured in hertz (Hz). A frequency meter is specifically designed to measure this parameter accurately. It works by detecting the periodic nature of the alternating current and displaying the frequency of the oscillations directly.

In contrast, other instruments like an ammeter measure current, a voltmeter measures voltage, and an ohmmeter assesses resistance. While all of these tools are essential for analyzing electrical circuits, they do not provide information about the frequency of the AC signal. Therefore, a frequency meter is the appropriate device for this purpose, highlighting why it is the correct answer.