I like your like of reasoning @asad24, and that certainly does seem like a valid retort to answer D, although for a weaken question the right answer does not have to be perfect, it just has to undermine the reasoning in the passage. Since the passage establishes that we no not need pain assessments since humans can make their own decisions, D points out that not all humans are indeed able to make their own decisions.
As to your more general question, we do not have to take answer choices literally as we do passages, but for strengthen and weaken questions an answer may not be airtight but would still clearly undermine or support the argument
