The first important labor union in the United States was called the

a. United Coal Miners.

b. Congress of Industrial Organizations.

c. Knights of Labor.

d. American Federation of Labor.

The first important labor union in the United States was called the Knights of Labor. The correct option among all the options given in the question is option "c". This labor union was formed during the late 1880's. Although this union did not survive for a long time due to lack of leadership, still it was a force to reckon with when it first started.