Final answer:
The erosion effect refers to a decrease in sales of certain products due to the expansion of other product lines.
Step-by-step explanation:
The erosion effect described in the question refers to a decrease in sales of certain products due to the expansion of other product lines. In this case, Luke expects an increase in fishing and hunting sales and a decrease in camping gear sales. The anticipated results would be a decrease in model train sales, which is III, and an increase in fishing and hunting sales, which is I. Therefore, the correct answer is c. I and III only.