What is an organization of workers united to protect and promote their common interests?

A labor union is an organization of workers who come together to advocate for their collective interests, particularly in matters concerning working conditions, wages, benefits, and workplace rights. The primary purpose of a labor union is to negotiate on behalf of its members with employers to establish fair labor practices and to provide a platform for workers to voice their concerns and needs. This collective bargaining power increases the influence of individual workers, as they are united in pursuit of better employment conditions.

Trade associations primarily focus on the interests of businesses and industries rather than individual workers. They advocate for the needs of companies within a specific sector, thus differing from the labor union's focus on worker rights.

Professional guilds generally cater to specialized professions and focus on standards and practices within specific fields, including certification and professional development. While they may also work collectively, it is not the primary focus on protecting workers' interests in a labor context like labor unions do.

Worker cooperatives are businesses owned and operated by their members, who are also the workers. While they do promote shared interests, they represent a different model than labor unions, which are specifically focused on negotiating and protecting workers' rights within existing employment structures rather than forming a business entity.

Hence, a labor union stands out as the correct answer