What should the 'pop()' method return when the stack is empty?

The programming-by-contract style would be that having a non-empty stack is a precondition of calling pop, and that calling a method without meeting its preconditions has an undefined outcome. My implementation would throw a std::logic_error, but that would not be required. In C, my implementation would abort via assert.

The caller of popis responsible for ensuring that the precondition that the stack is not empty holds before calling pop. The stack should therefore have an isEmpty method for the caller to check.

The C++ STL actuall doesn't return anything via pop() since it decouples returning the value of an object and actually popping an object off the stack's internal data-structure, making them two separate functions. So that's another option to consider in your design of a stack data-structure.

Your third option is also a pretty idiomatic approach for these types of data-structures.

For your fourth option, rather than a "unique empty element", I would actually do a variation on your third option where your pop() function that takes a pointer argument rather than a reference type, and returns NULL if there are no objects left in the stack.