(a)
Given ,$s = \sqrt{t}$
$\therefore\ \ \frac{ds}{dt} = \frac{1}{2\sqrt{t\ }}\ \ \ \ \ \ \Rightarrow v = \frac{1}{2\sqrt{t\ }}$
$\Rightarrow \ \frac{dv}{dt} = - \frac{1}{2.2t^{3/2}}$ $\Rightarrow a = \frac{- 2}{\left( 2\sqrt{t} \right)^{3}}$
$\Rightarrow \ \ \ a = - 2v^{3}\ \ \Rightarrow a \propto v^{3}$