Right choice is (b) -(G*M)/a
Explanation: We know that the gravitational potential at a point “p” at a distance of “r” from the centre of the ring of radius “a” and mass M” is;
V = -(G*M)/(a^2+r^2)^1/2
At the centre of the ring; r = 0.
Therefore; V = -(G*M)/a.