To make the cab company's charges a nonproportional situation, they could implement a flat rate charge in addition to the per-mile charge. The best option that reflects this change would be:
Charge a flat rate of $10 and then $8 per mile.
This would mean that customers have to pay a fixed amount ($10) regardless of the distance, plus the cost that varies with the number of miles traveled ($8 per mile). In contrast, the original pricing system was purely proportional since the cost varied directly with distance (0 miles cost $0, 1 mile cost $8, etc.). By adding a flat fee, the total cost will not be zero when the distance is zero, thus making the situation nonproportional.