Which of the following demonstrates one-to-one correspondence?

• A. Grouping items into sets by color.
• B. Counting each item while pointing to it and saying its number aloud.
• C. Placing two items into a single category.
• D. Matching each item in one set to a unique item in another set.

Answer: (D)

One-to-one correspondence means pairing each element of one set with a unique element of another set, so every item has its own partner and no partner is used twice. Matching each item in one set to a unique item in another set fits this idea exactly: every item finds a distinct partner, and no two items share the same match.

Grouping by color is just sorting, not pairing. Putting two items into one category loses the idea of unique partners. Counting while pointing and saying a number labels each item with a unique count, but that describes counting rather than establishing a pairing between two actual sets.