A tuning fork A produces 4 beats/s with another tuning fork B of frequency 320 Hz. On filing one of the prongs of A, 4 beats/s are again heard when sounded with the same fork B. Then, the frequency of the fork A before filing is

• 328 Hz

• 316 Hz

• 324 Hz

• 320 Hz

D.

320 Hz

There are 4 beats between A and B, therefore the possible frequencies of A are 316 or 324 that is (320 ± 4)Hz. 

When the prong of A is filed, its frequency becomes greater than the original frequency. If we assume that original frequency of A is 324 then on filing its frequency will be greater than 324. The beats between A and B will be more than 4. But it is given that the beats are again 4, therefore, 324 is not possible. 

Therefore, required frequency must be 316 Hz. (This is true, because on filing the frequency may increase so as to give 4 beats with B of frequency 320 Hz)