Correct Answer - `(Q)/(2 sqrt(2))`
Electric field at point P due to charge of ring is
`E=(kQx)/((R^(2)+x^(2))^(3//2))`
At `x=R, E=(kQ)/(2 sqrt(2)R^(2))` directed toward the center
Electric fild at P due to charge at center , `kq//R^(2)`.
For net field to be zero at P
`(kq)/(R^(2))=(kQ)/(2 sqrt(2)R^(2))` or `q=(Q)/(2 sqrt(2))`.