Final answer:
Reference groups have the strongest influence on the purchase of running shoes (c) because they are socially visible products and are often subject to peer influence and fashion trends.
Step-by-step explanation:
The influence of a reference group would be strongest for c. running shoes. This is because items like running shoes are often visible to peers and are subject to social influence and peer pressure. Reference groups such as friends, family, or celebrity endorsers can significantly impact consumer behavior, particularly in fashion and lifestyle product categories like running shoes. In contrast, products like vitamins, toilet tissue, insurance, and clothes washers are less visible and socially driven, hence less likely to be influenced by reference groups.