Correct Answer - b
Two opposite charges situated at opposite vertices forms a dipole and point lies on bisector line of dipole for on dipole
`vec (E )= (-Kvec(P))/(x^(3))`
direction of field due to these three dipoles is in a horizontal plane at distance `x` from plane of hexagon and parallel to it, mutually having angle `60^(@)` with each other.
`E_(0)= E+E=2E= 2(Q(2a))/(4piepsilon_(0).x^(3))= (Qa)/(pi epsilon_(0)x^(3))`
