If $x=at^2 $ and $y = a^3t^3 ,$ then $\frac{d^2y}{dx^2}$ will be :

$\frac{3at}{2}$ $\frac{3}{4at}$ $\frac{3a}{4t}$ $\frac{3a^2t}{2}$

$\frac{3a}{4t}$