The average of the three variables is (a + b + c)/3. However, we need to solve in terms of a, which means we must convert b and c into something in terms of a.

We're told that a = (1/4)b, which is equivalent to b = 4a. We can plug that in and simplify the average to: (a + 4a + c)/3

We also know that c = 7a, which we can plug directly into the average expression:

(a + 4a + 7a)/3

= 12a/3 = 4a, choice (C).