Technically, answers A, C and D are all partially correct, but the answer that is probably considered correct is A.
The primary function of the lipids (phospholipids) in a cell membrane is forming a protective layer. The analogy that I usually use is that they're like the bricks in a wall, whereas transport and channel proteins are like doors in that wall. So, the lipids protect the cell by forming a physical barrier to outside contents that cannot pass through the membrane itself (although some modern research suggests that things can pass directly through the membrane, but not very quickly or efficiently, so they still do a good job of keeping things out).
You could probably argue that the membrane is also involved in transport, because it helps to regulate the flow of materials in and out of the cell by creating the barrier that requires transport in the first place. As long as the membrane maintains a cohesive structure that is stronger than the pressure of the molecules inside and outside of it, it will provide the conditions under which regulated transport can occur.
The lipids themselves don't really do much in the way of communication, but they do provide the anchor points for the proteins which do the communicating, so in the same way that they provide a foundation for transport, they provide a foundation for communication as well.