If $x \sin (\alpha +y)=\sin y $ and $y'=\large\frac{m}{(x^2+2nx+1)}$, then

Question

$\begin{array}{1 1} (a) \;m-n=1 \\ (b) \;m+n=1\\ (c) \;m^2+n^2=1 \\ (d) \;m=n\end{array} $