The correct answer is (d) deny.
Word Analysis:
— deny: to declare that something is not true; to refuse to accept or acknowledge (অস্বীকার করা).
Sentence: 'No one can deny that he is clever' — অর্থাৎ কেউ এটা অস্বীকার করতে পারে না যে সে চালাক।
✗ defy (verb): to openly resist or refuse to obey (অমান্য করা, বিদ্রোহ করা) — 'No one can defy' would need an object that is a person or authority, not a fact clause.
✗ denounce: to publicly declare something as wrong or evil (নিন্দা করা).
✗ admire: to regard with respect and approval (প্রশংসা করা) — positive meaning, doesn't fit.
Source: Oxford Advanced Learner's Dictionary; BCS Previous Year Question Bank.