During World War II, the RAF was primarily responsible for which of the following?

The RAF's primary responsibility during World War II centered around the aerial defense of Britain and providing tactical support to the Army. This included engaging enemy aircraft to protect British airspace and conducting operations that supported ground troops in various campaigns.

During the Battle of Britain, for example, the RAF played a crucial role in defending against the Luftwaffe's attempts to gain air superiority over the UK. This operation was pivotal in shifting the momentum of the war in favor of the Allies. The RAF also engaged in a variety of ground support missions that aimed to disrupt enemy movements and provide assistance to Army operations.

While the RAF did contribute to strategic bombing campaigns, especially against German industrial targets, and played a role in some operations against Japan, their focused efforts and most notable achievements primarily revolved around the defense of Britain itself and aiding the Army, making this the correct choice. The development of new aircraft technologies, although important, was not their primary responsibility during the conflict.