(a)
(b)
(c)
The Commutative property of addition (and multiplication) states that the numbers we are dealing with in math can have their positions changed, without making any difference to the final answer.
In number 8(a), the fractions in parenthesis swapped places but this wouold not affect the answer.
The Associative property of addition (and multiplication) states that when two or more numbers are added (or multiplied) the results remain the same regardless of the change in grouping of the parenthesis.
In number 8(b), the first and second expression were grouped into parenthesis. But when the second and third expression were regrouped into the parenthesis, the answer would remain the same.