Of the three automobiles left on the lot, one was inexpensive, one was cheap, and one was economical. Which word describes the automobile that represents a prudent purchase?
The third option.
The first two merely tell you that it didn't cost much. However this does not mean it is a prudent buy as its low price may mean it is very expensive to run. However the third option clearly states that it is economical meaning or at least implying it has low running costs.