The correct answer is: Identity
Rationale: The definition of a group is, basically, 'any collection of people who perceive themselves to be a group' (Handy). Identity means that there are acknowledged boundaries to a group, which define who is 'in' and who is 'out', 'us' and 'them'. Groups may have a defined leader and purpose, but not always. Conformity can be an attribute of groups or crowds.
Pitfalls: Note that this wasn't a question about the difference between a group and a team - although this too would make a good exam question.
