From a large square of area a², two strips of area ab are removed and the small corner square of area b² is added back. What is the remaining area?

Correct answer: (a − b)²

The correct answer is A: (a − b)². Begin with a², the area of the large square. Two strips, each of area ab, are removed, so the temporary area is a² − ab − ab = a² − 2ab. The corner region of area b² belongs to both strips and was therefore removed twice; adding it back once gives a² − 2ab + b². By the identity (a − b)² = a² − 2ab + b², the remaining area is (a − b)². B and D both contain a positive 2ab term, which would describe adding the mixed rectangles rather than removing them. C, a² − b², is the difference-of-squares identity and has no −2ab term. The visual operation is subtraction with overlap correction, so A is unambiguous.