Find the equation of normal to the hyperbola $\large\frac{x^2}{a^2}-\frac{y^2}{b^2}$$=1$ at $(x_1,y_1)$ ?

Question

$\begin{array}{1 1}(a)\;\large\frac{b^2y}{y_1}+\frac{a^2x}{x_1}\normalsize = a^2+b^2\\(b)\;\large\frac{b^2y}{y_1}-\frac{a^2x}{x_1}\normalsize = a^2+b^2\\(c)\;\large\frac{b^2y}{y_1}+\frac{a^2x}{x_1}\normalsize = a^2-b^2\\(d)\;\large\frac{b^2y}{x_1}+\frac{a^2x}{y_1}\normalsize = a^2+b^2\end{array}$