Final answer:
The possible number of combinations of AA BB CC DD is 24 (option C).
Step-by-step explanation:
The possible number of combinations of AA BB CC DD is 24 (option C).
Each letter can be arranged in 4 different positions, resulting in 4! (4 factorial) combinations. The factorial of 4 (4!) is calculated as 4 x 3 x 2 x 1 = 24.
Therefore, there are 24 possible combinations of AA BB CC DD.