Theme is described as the central idea, lesson, or message that the author wishes to convey through a literary work.
As per the question, the theme of 'mortality' in the Act V, Scene I of play 'Hamlet' is clearly evident in the given lines as it reveals that humans cannot escape death as it is inevitable. This is substantiated through the claim that Alexander and Julius Caesar despite being such great man had to be 'buried and returneth to dust' and 'turn'd to clay' implying that all humans turn into dust eventually. This reflects the transient nature of human life and suggest that humans have to accept death, sooner or later. Therefore, option C is the correct answer.