Mary Pickford was known as "America's Sweetheart" primarily for her work in which industry?

Mary Pickford was primarily known as "America's Sweetheart" for her significant contributions to the film industry. She was a major star in the silent film era, captivating audiences with her roles that often portrayed youthful innocence and charm. Her performances and screen presence helped define the notion of a leading lady in early cinema, making her one of the most recognizable and beloved figures of the time.

During her career, Pickford co-founded United Artists, a film distribution company that allowed filmmakers greater creative control and profit-sharing, which further established her influence in the industry. This combination of her talent, business acumen, and the iconic characters she portrayed on screen solidified her reputation as a beloved figure and contributed to her enduring legacy in Hollywood history.

In contrast, the other options—theater, television, and literature—do not encapsulate her primary impact or public persona. While she did have some involvement in theater earlier in her career, it was her work in film that truly earned her the affectionate moniker and widespread fame.