Final answer:
A craft union is a type of labor union that represents workers who have a specific skill in a particular industry. The United Association of Plumbers is an example of a craft union as it represents specialized plumbers.
Step-by-step explanation:
A craft union is a type of labor union that represents workers who have a specific skill in a particular industry. Craft unions typically focus on securing benefits and protections for their members, such as higher wages, better working conditions, and job security. They often require that workers possess a specific skill or trade to be eligible for membership. Based on the options provided, the United Association of Plumbers would be considered a craft union because it represents plumbers who have a specialized skill.