To make the situation non-proportional, the company would need to change its pricing structure so that the total charge does not remain directly proportional to the distance traveled.
Charging a flat rate in addition to a per-mile charge introduces a non-proportional element, as the flat rate would apply regardless of the distance traveled. Therefore, the option that would create a non-proportional situation is:
Charge a flat rate of $20 and then $12 per mile.
This option incorporates a fixed cost, which breaks the direct proportionality of the cost to distance.