a
(a) \({n_x} = 300Hz,\) \({n_y} = ?\)

\(x =\) beat frequency \(= 4 Hz,\) which is decreasing \((4→2)\)

after increasing the tension of the string \(y. \)

Also tension of wire \(y\) increasing so \({n_y} \uparrow \)    (\( \because n \propto \sqrt T\))

Hence \({n_x} - {n_y} \uparrow = x \downarrow \) \(\rightarrow\) Correct

\({n_y} \uparrow - {n_x} = x \downarrow \) \(\rightarrow\) Wrong

==> \({n_y} = {n_x} - x = 300 - 4 = 296Hz\)