Final answer:
The rate that is proportional to the cost of a gallon of milk at Corry's Supermarket is the price per gallon.
Step-by-step explanation:
The rate that is proportional to the cost of a gallon of milk at Corry's Supermarket is the price per gallon. This means that the cost of milk is directly proportional to the quantity of milk you buy. For example, if you buy 2 gallons of milk, the cost will be 2 times the price per gallon. If you buy half a gallon of milk, the cost will be half of the price per gallon.