The correct choice is (c) ∞
The best I can explain: The potential due a dipole at a point P will be V = m cos θ/(4πεr^2).
Now it is given that potential on the midpoint, which means P is on midpoint, then the distance from midpoint and P will be zero. When r = 0 is put in the above equation, we get V = ∞. This shows that the potential of a dipole at its midpoint will be maximum/infinity.