Figure shows a charge q placed at the centre of a hemisphere. A second charge Q is placed at one of the positions A, B, C and D. In which position(s) of this second charge, the flux of the electric field through the hemisphere remains unchanged?

Explanation

If the charge is placed at A or D, any electric field line
will cross the curved surface twice.

hence, the flux passing through the curved surface
should be unchanged. If the charge is placed at B,
some of the electric field lines may cut the surface once
thereby changing the flux through the surface.