Let $T(x',y')$ and $T'(x'',y'')$ be the points of contacts of tangents drawn from $P(x_1,y_1)$ to $x^2+y^2=a^2$
Then equation of PT and PT' are $xx'+yy'=a^2$
Since both tangents pass through $(x_1,y_1)$ then
Point T and T' lie on
$\therefore$ Equation of chord of contact TT' is $xx_1+yy_1=a^2$
Hence (a) is the correct answer.