In which visual model do the parts combine to form a larger square with side a + b?

Correct answer: The model of (a + b)²

The correct answer is B: the model of (a + b)². In a visual square model, an a by a square, two a by b rectangles, and a b by b square can be placed together without gaps. The outside length in each direction is a + b, so the whole figure is a square with area (a + b)². Adding the four regions gives a² + ab + ab + b² = a² + 2ab + b². A represents a square of difference, whose outside side is a − b and whose mixed regions are subtracted. C represents a rectangle with sides a + b and a − b, not a larger square. D represents only one rectangular region with area ab. Thus B is the only model whose outer boundary has side a + b. Memory cue: read the outside boundary first; its side tells you which square identity is being modelled.