A visual model for a square of side (c−d) is made by starting with c² and removing two strips of area cd each. What final correction is needed to obtain the correct area?

Correct answer: Add d²

The correct answer is D: add d². Begin with a square of area c². Removing one strip of area cd gives c²−cd. Removing the second strip gives c²−2cd. But the two strips overlap in a small corner square of side d, whose area is d². That corner has been removed twice, although it should be removed only once. To correct this double subtraction, add d² once. Thus the result is c²−2cd+d², which is (c−d)². Option A is wrong because d² is not subtracted at the correction stage; it must be restored. Option B restores only cd, not the doubly counted corner. Option C subtracts 2d² and makes the area even smaller. Memory cue: in a difference-of-squares picture, the overlap removed twice is added back once.