What group did the first female Marine officer belong to upon commissioning?

The first female Marine officer, Second Lieutenant Annie L. Grimes, was commissioned into the Marine Reserve (F) upon her completion of training. The Marine Reserve (F) was specifically established for women who wanted to serve in the Marine Corps during World War II, allowing them to take on various roles to support the war effort. This marked a significant milestone in Marine Corps history, as it opened the door for women to officially serve in a capability that was previously reserved for men.

The other groups listed either did not pertain to the Marine Corps specifically or were entirely separate organizations. The Women's Army Corps, for instance, was associated with the Army, while the Women's Airforce Service Pilots were connected to the Air Force. The US Naval Reserve, while related to naval services, did not specifically cater to the unique needs and roles women would take on within the Marine Corps. Therefore, the choice reflecting the specific organization for female Marines at the time, the Marine Reserve (F), is the correct answer.