Final answer:
The synonym for 'pestilential' is 'harmful,' as it describes something that is noxious or capable of causing disease.
Step-by-step explanation:
The word pestilential is an adjective often used to describe something that is harmful or noxious, specifically relating to something that is capable of causing a plague or epidemic disease. Therefore, the synonym for "pestilential" out of the options provided is B. harmful. This is because the other options, delightful, pleasing, and pleasant, all have positive connotations, whereas pestilential has a decidedly negative connotation, aligning more accurately with something that is harmful or destructive.