(d)
Let coordinates of $O$ and $A$ (0, 0 ) and $\left( at^{2},\ 2at \right)$ respectively
$\therefore$ Coordinates of mid point of$\ OA\$ are
$\left( \frac{0 + at^{2}}{2},\frac{0 + 2at}{2} \right) = \left( \frac{at^{2}}{2},\ at \right)$
Since , $\left( at^{2} \right) = 4\ \left( \frac{at^{2}}{2} \right)$
Hence, that locus of required point is $y^{2} = 2x$