The associative property regroups values by changing the location of parentheses. If you notice, the order of the values is not changed. Both sides have a x b x c, only the parentheses have been moved. Moving the parentheses changes the progression of what is multiplied (whatever is in the parentheses is multiplied together first) but doesn't change the final answer.
The associative property of multiplication (or addition) shows that the order the numbers are multiplied it irrelevant, more specifically in the manners of parentheses.
So, the answer is A, which can also be seen in that substituting a as 5, b as 3, and c as 6 gives us the first example.
