Explanation

(c)
We know that, the locus of point of intersection of two perpendicular tangents drawn on the ellipse is $x^{2} + y^{2} = a^{2} + b^{2}$ , which is called director circle
Given equation of ellipse is $\frac{x^{2}}{9} + \frac{y^{2}}{4} = 1$
Here, $a^{2} = 9,\ b^{2} = 4$
$\therefore$ Locus is $x^{2} + y^{2} = a^{2} + b^{2}$
$\Rightarrow x^{2} + y^{2} = 9 + 4$
$\Rightarrow x^{2} + y^{2} = 13$