Properties of the multiplication
The associative property states that the product of three factors can be grouped in any order and they will produce the same result.
The statement
(3x4)x7 can be written in two equivalent forms by associating another pair of elements:
3x(4x7)
(3x7)x4
Selecting the first form:
(3x4)x7 = 3x(4x7)
Thus the correct choice is:
a. Associative