Which organization signed an agreement to support the AFM in 1913?

The correct choice is the Actor's Equity Association, which signed an agreement to support the American Federation of Musicians (AFM) in 1913. This partnership represented a significant step in the collaboration among various labor unions within the entertainment industry, aiming to strengthen their collective bargaining power and improve working conditions for artists and performers. The Actor's Equity Association specifically represents actors and stage managers in the theater industry, and their support for musicians highlighted a shared goal of advocating for fair treatment and rights within the entertainment sector.

Other organizations mentioned, such as the Screen Actors Guild and the American Federation of TV and Radio Artists, were established later and focus on different areas of the entertainment industry. The International Alliance of Theatrical Stage Employees also has a different focus, primarily revolving around technical aspects of theater and film production. Therefore, the milestone agreement in 1913 primarily involved the Actor's Equity Association, aligning with the historical context of labor unions advocating for artists in the performing arts.