Which of the following is a unit of frequency?

The correct choice is Hertz, which is the standard unit for measuring frequency in the International System of Units (SI). Frequency refers to the number of cycles of a periodic event that occur in one second. Thus, when we say that a signal has a frequency of 1 Hertz (Hz), it means that the signal undergoes one complete cycle per second.

The other units mentioned are associated with different physical quantities. For instance, Ohm is the unit of electrical resistance, Joule is the unit of energy, and Volt measures electrical potential difference. Each of these units represents a different aspect of electrical and physical phenomena, whereas Hertz specifically quantifies how often a repeating event occurs over time, making it the correct answer in this context.