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So not understanding this question from a conditional standpoint. It seems like the right answers makes a mistaken negation??

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Using DeMorgan's law: from the premise we have If A and B then C; the contrapositive is If not C then Not A or Not B; Put it in the argument here: If inviting and functional, then it must be not obtrusive; the contrapositive is if it is obtrusive, then it must be not inviting or not functional.\

From the premise we know the modern architects's work is not functional, then it must be obtrusive, which is exactly what choice B saying.