Answer: The correct answer is 5 Hz.
Step-by-step explanation:
The expression for the relation between the speed and wavelength is as follows:

Here, v is the speed of the wave, f is the frequency of the wave and
is the wavelength.
In the given problem, a wave is moving at the rate of 40 cm/s. Its wavelength is 8 cm.
Put v= 40 cm/s and
= 8 cm.


Therefore, the correct option is (A).