Question

the instrument used to study the laws of vibrating string is -
a) hydrometer
b) hygrometer
c) sonometer
d) electrometer

Answer 1

Sonometer is the instrument used to study the laws of vibrating string

Answer 2

c) Sonometer
A Sonometer is a device for demonstrating the relationship between the frequency of the sound produced by a plucked string, and the tension, length and mass per unit length of the string.

Hydrometer is used to measure specific gravity of a liquid
Hygrometer is used to measure water vapour in atmosphere
Electrometer is used to measure electric potential difference in an electrical circuit.