Which set of four regions is correct in the area model of a square with side (x+10)?

Correct answer: x^2, 10x, 10x, 100

The direct answer is A. Split each side of the square into lengths x and 10. The resulting four rectangular regions have areas x times x=x^2, x times 10=10x, 10 times x=10x, and 10 times 10=100. Their sum is x^2+10x+10x+100=x^2+20x+100, which is (x+10)^2. B combines the two equal strips into one expression, so it does not list four separate regions as asked. C incorrectly gives one strip as 100x, even though one dimension of that strip is 10. D makes both strips too large and gives the corner the wrong value. Memory cue: in a square split into x and 10 on both sides, expect one x^2 region, two identical 10x regions, and one 10^2 corner.