To make the charging method non-proportional, the company could implement a structure where the cost does not directly scale with the distance traveled.
The correct option is:
Charge a flat rate of $10 and then $8 per mile.
This structure means that there is an initial flat fee (the $10) that does not depend on the distance, making the overall charge non-proportional to miles driven since the first part (the flat rate) is constant regardless of distance.