AS  

V=KQ/R,

ACC. TO QUES.
1/2V=KQ/R
V=2KQ/R,

AS K AND Q ARE CONSTANT FOR A GIVEN CHARGED BODY, R SHOULD BE 1/2

HENCE, ANS IS "R/2"

Potential at the centre of the sphere is 3kq/2R, where R is the radius of the sphere and k=9*10⁹ . 

Let the distance of the required point from the surface of the sphere be x.

Then,

    3kq/4R=kq/x+R

Solving,

    x=R/3

Hence option C is the correct option....