d
(b) \(n_A\) = Known frequency \(= 288\, cps,\) \(n_B = ?\)
\(x = 4 \,bps\), which is decreasing (from \(4\) to \(2\)) after loading i.e. \(x\downarrow\)
Unknown fork is loaded so \(n_B\downarrow\)
Hence \(n_A -n_B\downarrow = x\downarrow\) \(\rightarrow\) Wrong
\(n_B\downarrow -n_A\downarrow = x\downarrow\) \(\rightarrow\) Correct
\(n_B = n_A + x = 288 + 4 = 292 Hz.\)