The frequency of AC mains in India is:
(A). $30\text{ Hz}$
(B). $50\text{ Hz}$
(C). $60\text{ Hz}$
(D). $120\text{ Hz}$

Hint: The frequency of AC or alternating current is the number of cycles that the current completes in one second. The mains frequency is the frequency of the AC current that reaches our homes and we use for all our purposes. Most European countries along with India also have the frequency of their AC mains as $50\text{ Hz}$.

Complete step by step answer:
The frequency of AC or alternating current is a measure of the number of total cycles that the sinusoidal variation of the current completes in one second. Since, in one cycle, the current changes direction twice, this means that in one second, the alternating current will change its direction twice the number of its frequency.
In general, European countries use $50\text{ Hz}$ as the frequency for their AC mains. Indian also follows this and similarly, also uses $50\text{ Hz}$ as the frequency for its AC mains. This means that the alternating current changes direction twice the frequency, that is $\left( 50\times 2=100 \right)$ times in one second. This is the AC that reaches our homes and we use it for all our daily purposes ranging from running fans to charging our mobiles.
Hence, the correct option is B) $50\text{ Hz}$.

Additional information:
The United States of America uses $\text{60 Hz}$ as the frequency for its AC mains.

Note: It might be of interest to students to know that the impedance of a circuit largely depends on the frequency of the AC mains. Hence, it is essential that there is no change in the frequency of the AC mains. All electrical appliances and products developed for the Indian market are configured in such a way so that they work optimally at $50\text{ Hz}$ frequency of AC. Hence, students might have also noticed that plug points in the USA and India are of different shapes.