Explanation

(c)
Let $P\left( at^{2},2at \right)$ be any point on the parabola $y^{2} = 4ax$, then equation of tangent and normal at $P\left( at^{2},2at \right)$ are $ty = x + at^{2}$ and $y = - tx + 2at + at^{3}$ respectively
Since, tangent and normal meet its axis at $T$ and $G$
$\therefore$ Coordinates of $T$ and $G$ are $\left( - at^{2},\ 0 \right)$ and $(2a + at^{2},\ 0)$ respectively
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From definition of parabola
$SP = PM = a + at^{2}$
Now, $SG = VG - VS = 2a + at^{2} - a = a + at^{2}$
And $ST = VS + VT = a + at^{2}$
Hence, $SP = SG = ST$