b
(b) $n_A $= Known frequency $= 256 Hz,$ $n_B = ?$
$x = 4\, bps$, which is decreasing after loading (i.e. $x\downarrow$)
also known tuning fork is loaded so $n_A\downarrow$
Hence $n_A\downarrow-n_B = x\downarrow $... $(i)$ $\rightarrow$ Correct
$n_B -n_A\downarrow = x\downarrow$ ... $(ii)$ $\rightarrow$ Wrong
==>$ n_B = n_A -x = 256 -4 = 252 Hz.$