Correct Answer (Detailed Explanation is Below)
A.
It is used to show that one problem can be transformed into another efficiently
Detailed Explanation
A
polynomial-time reduction transforms instances of one problem into instances of another problem using polynomial time. Reductions are central to proving NP-Completeness. If problem A can be reduced to problem B, an efficient algorithm for B can be used to solve A efficiently. To prove that a problem is NP-hard, researchers commonly reduce a known NP-Complete problem to the new problem. This direction is important: showing
A reduces to B indicates that B is at least sufficiently powerful to solve A through the transformation.