Final answer:
Marty kept the food cold using an icebox, a 19th-century appliance that used ice to preserve food before modern refrigeration became widespread.
Step-by-step explanation:
Marty kept the food he bought from Mr. Wallace cold using an icebox, which is the correct answer to the question. During the 19th century, an icebox was a common household appliance for keeping foods cold. The icebox operated by using large blocks of ice, which is illustrated in Figure 5.2, to maintain a cool temperature inside the box, preserving the food contained within.
The reference to the icebox implies a historical context, placing the scenario before the widespread adoption of modern refrigeration technology that we are familiar with today, such as refrigerators and freezers. This technology came into widespread use in the middle of the 20th century as outlined in the information provided.