Final answer:
The individual in a family who is responsible for the physical act of buying a product is known as the purchasing agent, distinct from other family decision-making roles such as the decision maker, influencer, gatekeeper, and consumer.
Step-by-step explanation:
In the context of family decision-making roles, the individual who performs the physical act of buying the product is referred to as the purchasing agent. This role specifically involves the transactional aspect of the decision-making process, which is the act of purchasing itself. This is different from the decision maker, who makes the final decision on whether or not to buy, the influencer, who affects the decision with their opinions, the gatekeeper, who controls information or access to products, and finally the consumer, who ultimately uses the product.