Final answer:
The situation described illustrates the law of demand, which states that there is an inverse relationship between price and quantity demanded. Therefore, the correct option is c. Law of Demand.
Step-by-step explanation:
The situation described, where Bart and Milhouse are willing and able to buy fewer comics as the price of the comics goes up, illustrates the law of demand. According to the law of demand, there is an inverse relationship between price and quantity demanded. As the price of a good or service increases, the quantity demanded decreases, assuming all other factors remain constant.