I would say sorrowful due to the context, however difficult would also make sense. As it is referring to the fault as his downfall I would stick with 'sorrowful' as the answer.
The answer is actually severe. It can’t be difficult, because it doesn’t make sense in the context of the statement. Nor could it be sorrowful, because at the moment Antony is the only one who see’s his death as such. Nor would it be political, because Caesar’s death in the play isn’t entirely politically driven. Therefore only sever could make sense, because it matches the context of the sentence, the tone of the speech, AND the plot of the play Julius Caesar.
