Find the slope of the normal to the curve $ x = 1 - a \sin\theta, y = b \cos^2\theta$ at $ \theta = \Large {\frac{\pi}{2}}$

Question

$\begin{array}{1 1}(A)\;\frac{2b}{a} \\(B)\;\frac{-2b}{a} \\ (C)\;\frac{-a}{2b} \\ (D)\;\frac{a}{2b} \end{array} $