Which organization was formed to unite skilled workers into national unions?

The American Federation of Labor (AFL) was established in 1886 primarily to unite skilled workers into national unions, aiming to improve their working conditions, wages, and job security. This organization focused on creating a higher level of bargaining power for skilled laborers by organizing them into craft unions based on specific trades. The AFL emphasized the importance of skilled labor and sought to negotiate directly with employers, rather than engaging in broader political movements or radical actions, which made it particularly effective at the time.

In contrast, the Knights of Labor sought to unite all workers, regardless of skill, gender, or race, into a single inclusive organization, which is why it did not focus exclusively on skilled labor. The Industrial Workers of the World (IWW), on the other hand, was established in 1905 and was known for advocating for industrial unionism and was more radical in its approach to labor issues, which differed from the AFL's methods. Lastly, the Trade Union Congress is a collective organization for trade unions in the UK and does not pertain to the unification of skilled workers in the United States.

Thus, the AFL’s specific focus on skilled workers and its strategy of forming national unions makes it the correct choice for this question.