Explanation

(c) Let $m$ be mass of planet and $M$ that of sun, $r$ the radius between the two. Let the planet be moving with velocity $v_{o},$ then
Gravitational force = centripetal force
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$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{GMm}{r^{2}} = \frac{mv_{o}^{2}}{r}$
$\Longrightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ v_{o} = \sqrt{\frac{GM}{r}}$
$\Longrightarrow \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ v_{o} \propto \frac{1}{\sqrt{r}}$
Hence, larger the distance, smaller the orbital velocity. At point $C$ distance from sun is maximum, hence orbital velocity is lowest. At point $A$ distance from sun is minimum, hence orbital velocity is maximum.