a
(a) Suppose $ n_A$ = known frequency $= 100 \,Hz, \,n_B\, = ?$
$x = 2 =$ Beat frequency, which is decreasing after loading (i.e.$x\downarrow$)
Unknown tuning fork is loaded so $n_B \downarrow$
Hence $n_A -n_B \downarrow = x\downarrow$ ... $(i)$ $\rightarrow $ Wrong
$n_B\downarrow -n_A = x\downarrow$ ... $(ii)$ $\rightarrow $ Correct
==> $n_B = n_A + x = 100 + 2 = 102 Hz.$