a.

Communication Diagrams show attributes of the objects, while Sequence Diagrams show the ordering of messages

b.

Communication Diagrams are mandatory to have sequence numbers, while in Sequence Diagrams, they are optional

c.

Communication Diagrams show the entities and their communication, while Sequence Diagrams show the ordering of messages

d.

Communication Diagrams show the objects and their relationships, while Sequence Diagrams show the attributes of the objects