A UML state transition diagram does not specify a set
of object that work together to perform some action, because
this is what a sequence diagram does instead. Conversely, the
second one is correct. As we said, a state transition diagram
describes the events that cause a transition from one state
to another. Again, a UML state transition diagram does not
specify the set of components in a system, because this
is what a component diagram does, not a state transition diagram.
As for the last one, this is correct,
because, as we also discussed, a state transition diagram
describes the actions that result from a state
change, that is, the effects of such state change.