Answer: (c) 60 vibrations per minute
Hertz is the unit for frequency. Frequency is the number of cycles undergone during one unit of a time by a body in periodic motion.
The SI unit of frequency is hertz (Hz). The term has been kept in honour of popular German physicist Heinrich Hertz. Generally, when we say one hertz it means that an event reoccurs once per second.
1 hertz is = 1 vibration per second
So 1 hertz is = 60 vibrations per minute