The unit of frequency is:-
A. Hertz
B. Joule
C. Ohm
D. Kilocalorie

Hint:First of all we should know about what is frequency, Hence, Frequency, in physics, is the number of waves that pass a fixed point in unit time; also, the number of cycles or vibrations undergone during one unit of time by a body in periodic motion. It is also defined as the number of rotations completed by a body in one second and time period is defined as the time taken by an object to complete one rotation.

Complete answer:
A body in periodic motion is said to have undergone one cycle or one vibration after passing through a series of events or positions and returning to its original state.Frequency is the rate at which current changes direction per second. It is measured in hertz (Hz), an international unit of measure where 1 hertz is equal to 1 cycle per second. Usually frequency is expressed in the hertz unit, named in honour of the 19th-century German physicist Heinrich Rudolf Hertz

Hence, the option A is correct, which is Hertz.

Note:The symbols most often used for frequency are f and the Greek letters nu ($\nu $) and omega ($\omega $).Nu is used more often when specifying electromagnetic waves, such as light, X-rays, and gamma rays. Omega is usually used to describe the angular frequency—that is, how much an object rotates or revolves in radians per unit time.