Which of the following expressions shows that rational numbers are associative under multiplication

The question is incorrect.

The correction required is in option1 ,infront of it should be either (-) sign or ( + ) sign.

Let only take one case of ( + ) sign infront of .

⇒The equation now becomes

⇒This equation follow associative property of rational numbers i.e.

a + (b + c) = (a + b) + c.

⇒Hence,(a) is answer.

Other options are irrelevant they do not follow the associative property.