To make the cab company's charges a nonproportional situation, they would need to change the structure of their pricing so that the relationship between the distance traveled and the total cost is not consistent.
In this case, the best option provided is:
Charge a flat rate of $10 and then $8 per mile.
This introduces a fixed cost (the $10 flat rate), which means that the total charge will no longer be directly proportional to the number of miles traveled. The other options either maintain a proportional relationship or change the cost per mile without adding a fixed component.