Which of the following began his career as a leader of the American Railway Union, was jailed after the Pullman strike, and later headed the Socialist Party of America?

Powderly was the leader of the Knights of Labor, a different labor organization, and was not directly involved with the American Railway Union. Although he advocated for workers' rights, he did not have the same involvement in the Pullman strike or the Socialist Party as Debs did.

Gompers was the founder of the American Federation of Labor (AFL) and focused on organizing skilled workers rather than leading the American Railway Union. He did not have a direct connection to the Pullman strike or the Socialist Party in the same way Debs did.

Haywood was a leader of the Industrial Workers of the World (IWW) and was involved in various strikes and labor movements, but he did not lead the American Railway Union or have a significant role in the Pullman strike, nor did he head the Socialist Party.

Thomas was a later leader of the Socialist Party of America but did not have the same early involvement in the labor movement or the events surrounding the Pullman strike as Debs, who was pivotal in these early struggles.