Baneful means something that is harmful or destructive.
“Harmful” means something that is most likely to cause harm, hence Option D is the correct answer because it is the most suitable meaning of the given word in italics.
Option A is incorrect because “unpleasant” means something that is not pleasant or soothing which is not relevant to the given word.
Option B is incorrect because “foul” means unpleasant or offensive to the senses hence it is not related to the given word.
Option C is incorrect because “pertinent” means relevant to a particular matter, which is not the correct meaning for the given word.