Find the equation of chord of contact of tangent drawn from a point $(x_1,y_1)$ to the circle $x^2+y^2=a^2$?

Question

$\begin{array}{1 1}(a)\;xx_1+yy_1=a^2\\(b)\;xx_1-yy_1=a^2\\(c)\;xx_1+yy_1=a\\(d)\;xx_1^2+yy_1^2=a^2\end{array}$