The correct option is C Zero
Since any point on the equatorial line of a dipole is equidistant from the positive and negative charge, the potential at any point on it is zero. So on moving any charge along the equatorial line the net work done by the field and therefore external agent is zero.
W.D=q(Vf−VI)=q(0−0)=0
In fact if you consider the entire plane lying midway between the dipole you will get the same result. This plane is an equipotential surface as no work is done by electric force in moving any charge in that plane.