SOLUTION —
$\because x \cos \theta+y \sin \theta=\cos \theta \cos \alpha-\sin \theta \sin \alpha$
On comparing $\cos \theta$ and $\sin \theta$, we get
$x=\cos \alpha$ and $y=-\sin \alpha$
$\therefore x^2+y^2 =\cos ^2 \alpha+(-\sin \alpha)^2=1$
$y =-\sin \alpha$
$\Rightarrow \alpha =\sin ^{-1}(-y)=-\sin ^{-1}(y)$
So, The correct option will be (B).