Explanation

(b)
The equation of given curve is $\sqrt{x} + \sqrt{y} = \sqrt{a}$
$\therefore\ \frac{1}{2\sqrt{x}} + \frac{1}{2\sqrt{y}}\frac{dy}{dx} = 0$
$\Rightarrow \ \ \frac{dy}{dx} = - \frac{\sqrt{y}}{\sqrt{x}}$
The normal is parallel to $x$-axis, if
$\left( \frac{dx}{dy} \right)_{\left( x_{1},y_{1} \right)} = 0\ \ \Rightarrow \ \ \ x_{1} = 0$
$\therefore$ From equation of curve, $y_{1} = a$
$\therefore$ Required point is $(0,\ a)$