The frequency in SI units is measured using
(A) $ {\text{Hertz}} $
(B) $ {\text{Hertz/sec}} $
(C) $ {\text{Hertz - sec}} $
(D) $ {\text{Hertz/oscillation}} $

Hint: To solve this question, we must look into the definition of frequency. Then we need to determine its dimensions and hence we can eliminate all the incorrect options that are provided in the question.

Formula used: The formula which is used to solve this question is given by
 $ f = \dfrac{1}{T} $ , here $ f $ is the frequency, and $ T $ is the time period.

Complete step by step solution:
We know that the frequency is defined as the number of oscillations produced per unit time. The time in which one oscillation is completed is termed as the time period. So we have one time period equivalent to one oscillation. By the unitary method, we get one unit of time equivalent to the reciprocal of the time period. So effectively we have the frequency equal to the reciprocal of the time period. Therefore
 $ f = \dfrac{1}{T} $
We know that the SI unit of time is seconds. So the SI unit of frequency becomes oscillations per second, or $ {\text{oscillations}}/\sec $ .
So, the options B and C are incorrect.
Also, as the frequency denotes the number of oscillations in a unit of time, so its unit can also be equal to oscillation.
So, the option D is incorrect.
Now, although $ {\text{oscillations}}/\sec $ is the correct SI unit of frequency, it is not used commonly. This unit is defined as Hertz. So, the SI unit of frequency is Hertz.
Hence, the correct answer is option A.

Note:
The unit Hertz, which is accepted as the SI unit for frequency, is actually named in honour of the German physicist Heinrich Rudolf Hertz, who was the first person to give a conclusive proof for the existence of the electromagnetic waves. These waves were originally predicted by the mathematical equations given by James Clerk Maxwell. This unit is generally used in the description of the sinusoidal waves. Also, it is used in the description of the clock speeds of various electronic devices.