In what year did Eleanore McMillen open her interior decorating firm?

Eleanore McMillen opened her interior decorating firm in 1924, marking the beginning of a significant career in the field of interior design. This year is notable not just for McMillen's contributions but also for being part of a broader movement in the early 20th century that saw the establishment of interior design as a recognized profession. Her work helped to elevate the standards and practices within the industry, paving the way for future designers.

The other years do not align with the documented history of her firm’s founding. While 1920 was an era that saw many budding design talents begin to flourish, it predates McMillen's entrance into the professional realm. The years 1930 and 1935 fall significantly later than the inception of her firm, thus highlighting the importance of the mid-1920s for the establishment of her brand and influence in interior design history.