Absolute value is defined as follows:
|x| = x if x ≥ 0 and -x if x < 0.
To make an equation using absolute value, you need to look for the following:
1) something where you use positive and negative numbers
2) something where changing the sign between positive and negative, or vice versa, doesn't affect the problem.
Choice A talks about price per gallons. You can't have negative money amounts so this one won't work.
Choice B talks about distances of walking. In math classes you can have negative distance, but not in reality. Nope.
Choice D talks about books. But books sold as a negative number means you bought them, and the meaning changes. Nope.
Choice C uses "plus or minus". It also talks about something that's approximate, and it's also a case where the signs won't matter. Bingo.
Thus choice C is best.