In the grid model of ((a+b+c)^2), what type of parts are (a^2), (b^2), and (c^2)?

Correct answer: Square parts

The direct answer is option B: square parts. In the area model, a large square has side length a+b+c. The horizontal and vertical divisions create smaller regions. The region with side a in both directions has area a × a = a^2, so it is a square. Similarly, the region with side b in both directions has area b × b = b^2, and the region with side c in both directions has area c × c = c^2. Option A is wrong because a rectangle has two possibly different side lengths, such as ab or ac; these mixed products occur in the model but are not the pure square terms. Option C describes a boundary line, not a two-dimensional area. Option D refers to the outside boundary, whereas a^2, b^2 and c^2 are interior areas. Memory cue: a variable multiplied by itself gives the area of a square whose side is that variable.