The figure shows an imaginary cube of length L/2 and a uniformly charged rod of length L touching the centre of the right face of the cube normally. At time t=0, the rod starts moving to the left slowly at a constant speed 'v'. the electric flux(F) through the cube is plotted against time(t). the correct graph showing the variation of flux with time is
Flux first increases linearly (∵ V is constant), remains constant (when length L/2 is inside the cube) and then decreases linearly (as rod moves out).