We can show that the clique problem is NP-hard by proving that :
Show answerHide answer
Correct answer: (D) None of the above
Explanation
To show CLIQUE is NP-hard, a known NP-complete problem must be reduced to CLIQUE (e.g. 3-CNF-SAT ≤ₚ CLIQUE), not the other way round. Reducing CLIQUE to 3-CNF-SAT, VERTEX_COVER or SUBSET_SUM only shows CLIQUE is no harder than them. So the answer is None of the above.