I would say to the interviewer a function object or functor is none of the four an that the first 3 can be modeled with with a functor.
1) Action
A function object can model an action or event. The function would contain details about how to process the action or attributes of the event. This is similar to may GUI's and callback methods work.
2) Predicate
A predicate is a function that returns a boolean value. Many STL
algorithms and containers use predicates. One example is for sorting.
3) Operator
A function object can be used as an operator. Many people only implement the operator()
method of a functor. A function object can be used to specialize an operation, such as addition.
4) Use Case
A Use Case is one scenario about how a task or program is used. It is an abstract concept usually used in determining the requirements of a system.
I have no problems letting interviews know they are wrong, either on tests or their verbal questions, as long as I have solid proof to back up my claims.