Correct answer - !Ref : The intrinsic function Ref returns the value of
the specified parameter or resource. When you specify a parameter's logical name, it returns the
value of the parameter, when you specify a resource's logical name, it returns a value that you
can typically use to refer to that resource such as a physical ID.
Incorrect:
!GetAtt - A function returns the value of an attribute from a resource in the
template
!Param - Not a valid function name
!Join - A function that appends a set of values into a single value, separated by
the specified delimiter