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.