Final answer:
Picketing by union members outside stores selling a manufacturer's products in order to exert pressure on the manufacturer is known as a secondary boycott.
Step-by-step explanation:
When union members are picketing outside retail stores that carry their apparel manufacturer's products, this is typically referred to as a secondary boycott. Unions use this tactic in an attempt to put pressure on the business they have a dispute with by involving a third party, which in this case, consists of the stores selling the manufacturer's apparel. This form of protest aims to discourage customers from purchasing the manufacturer's products, thereby causing economic pressure that can lead to the manufacturer addressing the union's demands, such as meeting with representatives, arguing for shorter hours, a living wage, and better working conditions.