Answer:
D. The frequency is not affected as the volume of sound increases
Explanation:
Volume of the sound is defined by the amplitude of the wave or how loud a sound is whereas Frequency is basically the pitch of the sound and is inversely proportional to the wavelength . Increasing frequency will increase the pitch of the sound without affecting it's volume.
To solve this problem it is necessary to apply the concepts related to frequency and vibration of strings. Mathematically the frequency can be expressed as

Then the relation between two different frequencies with same wavelength would be


The beat frequency heard when the two strings are sounded simultaneously is



We have the velocity of the transverse waves in stretched string as


And,

Therefore the relation between the two is,


Finally substituting this value at the frequency beat equation we have


Therefore the beats per second are 11.92Hz
To solve this exercise it is necessary to use the concepts related to Difference in Phase.
The Difference in phase is given by

Where
Horizontal distance between two points
Wavelength
From our values we have,


The horizontal distance between this two points would be given for

Therefore using the equation we have




Therefore the correct answer is C.