Explanation

(a)
Given, $\sin^{2}{A + \sin^{2}{B + \sin^{2}{C = 2}}}$
$\Rightarrow \ \ 1 - \cos^{2}{A + 1 - \cos^{2}{B + 1 - \cos^{2}{C = 2}}}$
$\Rightarrow \ \ 1 = \cos^{2}{A + \cos^{2}{B + \cos^{2}C}}$
$\Rightarrow \ \ 1 = 1 - 2\cos{A\cos{B\cos C}}$
$\Rightarrow \ \cos{Acos\ B\cos{C = 0}}$
At least one should be $90{^\circ}$ and sum of two angles should be $90{^\circ}$