Definition of One-to-One Correspondence is:

• A. Members of one set can evenly be matched to members of a second set.
• B. Objects can be counted without pointing.
• C. Two sets always have the same number of elements.
• D. A shape can be placed into another shape.

Answer: (A)

A one-to-one correspondence means every element in the first set has a unique partner in the second set, and every element of the second set is paired with exactly one element of the first set. This exact, unique pairing is why describing the situation as members of one set being evenly matched to members of a second set is the best fit. It captures the idea that there’s a perfect, bidirectional matching with no leftovers and no duplicates—each item on both sides is used once.

Think of it like assigning each student to a locker when there are exactly the same number of students and lockers: every student has their own locker, and no locker is shared. Counting objects or measuring quantity isn’t about pairing, and merely having the same number of elements is a consequence of such a perfect pairing rather than the definition itself. Placing a shape inside another shape is about containment, not about pairing elements across two sets.