Explanation

(d) Centripetal acceleration, $a_{c} = \frac{v^{2}}{R}$
Where $v$ is the speed of an object and $R$ is the radius of the circle
It is always directed towards the centre of the circle. Since $v$ and $R$ are constants for a given uniform circular motion, therefore the magnitude of centripetal acceleration is also constant. However, the direction of centripetal acceleration changes continuously. Therefore, a centripetal acceleration is not a constant vector