答案：D

解析：

Triangle A B C is congruent to triangle D E F, so the lengths of the three sides of triangle D E F are the same as the lengths of the three sides of triangle A B C. Triangle A B C is a 30 degrees - - 90 degrees triangle with hypotenuse of length 12, so the other two sides of triangle A B C have lengths 6 and 6 times (square root 3). Therefore, the lengths of the sides of triangle D E F must be 12, 6, and 6 times (square root 3). Of the choices given, only 6 times (square root 3) is one of these values.