A. Wasn't.. This is because if you seperatevthe contractioninto two words it is was not, this would make the sentence be "Wally was not capable of lifting the heavy box." If you were to do the same thing with weren't it would sound like "wally were not capable of lifting the heavy box." This does not make sense because "wally" is singular (so is "was not") and "were not" is pleural.
It is false that when Claudius leaves the play, everyone follows and only Hamlet is left on stage. As a matter of fact, some characters stay on stage with him even after Claudius' departure.
