a
(a) $nA = ?, nB =$ Known frequency $= 320 Hz$
$x = 4$ bps, which remains same after filing.
Unknown fork $A$ is filed so $nA\uparrow$
Hence $nA\uparrow -nB = x$ Wrong
$nB -nA\uparrow = x$ Correct
==> $nA = nB -x = 320 -4 = 316 Hz$.
This is the frequency before filing.
But in question frequency after filing is asked which must be greater than $316 Hz$, such that it produces $4$ beats per sec. Hence it is $324 Hz.$