Check the life of the variable. Since the variable s is a local variable , it does not exists outside the method and hence it should be eligible for garbage collection. Note that the question says it is eligle for garbage collection but not forcing the garbage collector to take the object.
s = s + r; The above line will produce a new String literal "abcnull" and s will be referring to this new object. The String object "abc" referred to by s previously will be eligible for garbage collection.
Note in the question it is given as "Before statement 4" and local variables become eligible for garbage collection only after statement 4.
Agree with Atul's comment. The question talks about the original object pointed to by s. Local variables are available for gc once the method ends - i.e NOT before line 4. Please correct me if otherwise.