d
(d) $n_A =$ Known frequency $= 341\, Hz,$ $n_B = ? $
$x = 6\, bps,$ which is decreasing (i.e. $x\downarrow$)
after loading (from $6$ to $1\, bps$)
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
$\Rightarrow$$n_B = n_A + x = 341 + 6 = 347 Hz.$