b
(b) $n_A$ = Known frequency $= 256 Hz, n_B = ?$
$x = 6\, bps$, which remains the same after loading.
Unknown tuning fork $F_2$ is loaded so $n_B\downarrow$
Hence $n_A -n_B\downarrow = x $... $(i)$ $\rightarrow$ Wrong
$n_B\downarrow -n_A = x$ ... $(ii)$ $\rightarrow$ Correct
==> $n-B = n_A + x = 256 + 6 = 262 Hz.$