Relative magnitudes of intermolecular forces from phase diagrams
This is an interesting situation. It would be easier to answer if the mp and bp of A were both either higher or lower than for B.

But your measure of the strength of intermolecular forces will be melting point - and boiling point. And they give different answers, so how do we make a decision? 

Consider the boiling point first: the higher boiling point of B suggests higher intermolecular forces, as you have concluded. Heat of vaporization is needed to put a molecule into the vapor phase, and it seems that B needs more energy to go gaseous.

But A melts at a higher temperature, suggesting that its molecules are held together in a solid phase by stronger bonds. Solid A needs more energy to turn it into a liquid; perhaps A can pack into a tighter solid structure. It would then need more heat to melt it, so it has a larger heat of fusion. 

Then your answer must have two parts: A is more tightly bound in a solid framework, but after you put enough heat into both A and B to melt them, B has more intermolecular bonding capacity as a liquid than A does. So B boils higher and is more strongly bound as a liquid.