c. their monthly cost of eating out at restaurants.
Variable expenses refer to expenses that change from time to time. An example of variable expenses is the monthly expenses for eating out in restaurants. This is because there is no defined number of times an individual should eat in a restaurant. Thus, the cost of eating in restaurants varies every month. Fixed expenses remain constant every month. Examples of fixed expenses include money spent on health insurance, rent payment, and car loans. Therefore, it is correct to argue that the right answer is option c.