Final answer:
The Card–Krueger study of the minimum wage rate is a natural experiment because it compared the effects of an increased minimum wage in New Jersey with no change in Pennsylvania, using similar border towns for the comparison.
Step-by-step explanation:
The Card–Krueger study of the minimum wage rate in New Jersey and Pennsylvania was considered a natural experiment because of the fact that the minimum wage increased in New Jersey, but not in Pennsylvania, allowing for a comparison in employment changes across similar fast-food restaurants in border towns. The correct answer to the student's question is option (d): The fact that the border towns in the two states were similar provided a natural setting for observing the effects of a minimum wage increase, without the need for a randomized controlled trial. This study is significant for policymakers and economists because it delves into the macroeconomic implications of minimum wage laws on employment without the possibility to carry out controlled experiments. Furthermore, the study considers the effects of the minimum wage increase as an exogenous variable, akin to the concept of natural experiments in other scientific fields.