Correct Answer - D
Potentials at the centre of ring are
`V_(0_1) = (k.q)/(R ) + (k(-q))/(sqrt(R^(2) +d^(2))),V_(0_(2)) = (k(-q))/(R ) + (kq)/(sqrt(R^(2) +d^(2)))`
`rArr V_(0_(1)) - V_(0_(2)) = 2kq [(1)/(R ) - (1)/(sqrt(R^(2) +d^(2)))]`
`= [(1)/(R)- (1)/(sqrt(R^(2) +d^(2)))]`