Explanation

(d)
Given curve is
$x^{2} + y^{2} = a^{2}$
$\therefore\ \ \ \ 2x + 2y\frac{dy}{dx} = 0$
$\Longrightarrow \ \ \ \ \ \ \ \frac{dy}{dx} = - \frac{x}{y}$
Given , $\frac{dy}{dx}$=0
$\Rightarrow \ - \ \frac{x}{y}$=0
$\Rightarrow \ x = 0$
$\therefore\ \ y = \pm a$
But $y \geq 0$given
$\therefore\ \ \ \ \ \ \ \ \ \ y = a$
$\Rightarrow Required\ point\ is\ (0,a)$
Alternate It is clear from the graph, tangent is parallel to x-axis for $y > 0$ is at (0,a)
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