(d) Given, equation is
$y = {9x}^{2}\ \ \ldots.(i)$
Since, $x$-component of velocity remains constant, we have
$\frac{dx}{dt} = \frac{1}{3}{ms}^{- 1}\ \ \ \ldots(ii)$
From Eq. (i), we have $y$-component of velocity.
$\frac{dy}{dt} = 18x \bullet \left( \frac{dx}{dt} \right)^{2}$
$\frac{dy}{dt} = 18\left( \frac{dx}{dt} \right)^{2} = 18 \times \left( \frac{1}{3} \right)^{2} = 2{ms}^{- 2}$
$\therefore\mathbf{a}_{y} = 2\ \widehat{\dot{\mathbf{j}}}\ {ms}^{- 2}$