The official solution is that A has branched / side chains, weak Van der Waals forces between the polymer chains, less compact packing than A, and large inter-chain distances. B has no branched chains, hydrogen bonds between the polymer chains, compact packing, and small inter-chain distances.
What does a branched chain mean? How does this influence the density? Doesn't B also have side chains?
What does 'compact packing' mean in this case? Does it mean that separate chains are packed closely together, or that in a single chain, the molecules are closer together?