The meaning of the question is to tell you how the area formula of this figure came from.

First of all, this is a square with a side length of (a+b).

The formula of square area is side length * side length, that is, (a+b)?

And there are four small squares in the picture. ab and B are in them? ，a？ Ab actually represents the area of four rectangles, namely, two rectangles with length and width of A and B, a square with side length of A and a square with side length of B..

The sum of the areas of four rectangles =ab+b? +a？ +ab=a？ +b？ +2ab=(a+b)？ = area of a square with a side length of (a+b)

So choose C.