What UML diagram types may be used to represent the essential features of a system?

What UML diagram types may be used to represent the essential features of a system?
Answer: (a) Activity diagrams, which show the activities involved in a process or in data processing.
(b) Use case diagrams, which show the interactions between a system and its environment.
(c) Sequence diagrams, which show interactions between actors and the system and between system components.
(d) Class diagrams, which show the object classes in the system and the associations between these classes.
(e) State diagrams, which show how the system reacts to internal and external events.