The correct answer is C. nuclear.
The primary stress in "ancient" (A), "exhilarating" (B), and "opportunity" (D) is on the first syllable, whereas in "nuclear" (C), the stress is on the first syllable too but it sounds different because of its distinct pronunciation pattern.
The correct answer is A. pursue.
The primary stress in "pursue" (A) is on the second syllable, while the primary stress in "popularity" (B), "anniversary" (C), and "promise" (D) is on the first syllable.