In the International System of Units (SI), which of the following units is used to measure the frequency of a periodic wave?

The correct option is Hertz.

Explanation:
Frequency is defined as the number of complete cycles, oscillations, or vibrations of a periodic wave that occur in one unit of time.

In the International System of Units (SI), the unit used to measure frequency is the hertz (symbol: Hz), named after the German physicist Heinrich Hertz. One hertz represents one cycle per second:

$1\text{Hz}=1{\text{s}}^{-1}$

Analysis of other options:
• Meter: The meter (m) is the SI unit of length or distance, used to measure spatial dimensions such as wavelength.
• Second: The second (s) is the SI unit of time, used to measure the time period of a wave cycle.
• Decibel: The decibel (dB) is a logarithmic unit used to measure sound intensity level or power ratio, not frequency.

Therefore, the SI unit of frequency for a periodic wave is the Hertz.