The passage opens up with the fact that a housefly is a nuisance. S will follow immediately as it mentions the reason why it is considered a nuisance. P further adds what many has considered them to be. R presents the current situation and it describes it as man’s worst enemy. Q tells the reason for it being the enemy for it carries the diseases along with it. S6 concludes the statement. SPRQ is the correct sequence. Hence, (d) is the correct option.