If $x=6 t, y=4 t^3$, then $\frac{d^2 y}{d x^2}$ at t = 1 is:

$\frac{4}{3}$ 2 $\frac{1}{3}$ $\frac{2}{3}$

$\frac{2}{3}$

The correct answer is Option (4) → $\frac{2}{3}$