Question

velocity of sound in a medium is 360m/s rarefactions separated by 1m

Answer 1

you should first understand about wavelength.
wavelength is defined as separation between two successive compression or separation between two successive rarefaction.

given, successive compression and rarefaction are separated by 1m

or, two successive compression or rarefaction/4 = 1m

or, wavelength = 4 × 1m = 4m

now use formula,
speed of sound = wavelength × frequency
360m/s = 4m × frequency
frequency = 90Hz

hence, option (D) is correct.

Answer 2

Given,

Distance between successive compressions and rarefactions = Wavelength = 1m
Velocity of sound = 360m/s
According to the formula,
Velocity = frequency *Wavelength
360=f*1
f=360Hz !!!ANSWER!!!

The question is easy but the catch is that:
Distance between successive compressions and rarefactions is also known as Wavelength.