答案：C

解析：

Choice (C) is correct. A square with sides of length 3 has area 9, and a square with sides of length 6 has area 36. Thus at most 36 divided by 9 equals 4 squares of side length 3 can fit inside a square of side length 6 without overlapping. And in fact, it is possible to fit the four squares of side length 3 inside a square of side length 6 with no overlap; if the four squares with sides of length 3 are arranged in two rows with two squares in each row, they will fit inside of the square with sides of length 6 without overlapping. Therefore, the maximum number of nonoverlapping squares with sides of length 3 that will fit inside of a square with sides of length 6 is four.