Final answer:
The commutative law of multiplication allows us to rewrite (9*12) 20 as (12*9) 20. Hence, a) is correct.
Step-by-step explanation:
The commutative law of multiplication states that changing the order of the numbers being multiplied does not change the product. In this case, we have (9 * 12) 20. Using the commutative law, we can rewrite this expression as (12 * 9) 20. So the correct answer is option a, (12*9) 20.