The squares here are organized in a way where one can prove the Pythagorean theorem. The Pythagorean theorem is the theorem that states that the length of one side of a right triangle, squared, plus the length of another leg of the triangle, squared, is equal to the hypotenuse squared. This is a² + b² = c². Since the areas of the squares are the squared lengths of the sides, that means that D. is the right answer.
