your correct answer would be, a.
because they thought that the desease came from crows
the phrase "star-cross'd lovers" introduces the theme of love and fate by mainly describing love from far away or something like that because in romeo and juliet they cannot see each-other because of their families, therefore making them star-cross'd lovers.
the phrase "death-mark'd love" introduces the theme of love and fate by explaining how they will love til' "death do us part." it also introduces the theme of the prologue because in the prologue they love each-other until death, which the reader infer what will happen in the story in a type of way.
i do sincerely hope this in many ways and you better understand the phases and relations to the books. : ) < 3