Which of the following movements is primarily facilitated by the deltoid muscle?

The deltoid muscle plays a crucial role in the abduction of the arm, particularly when the arm is raised to the side. This muscle is structured in three distinct parts: the anterior, lateral, and posterior fibers. The lateral fibers, in particular, are most significantly engaged during arm abduction.

While the deltoid also assists in other movements such as flexion and extension, its primary action is to lift the arm away from the body, making it the key muscle in achieving abduction. This action begins around 15 degrees of shoulder elevation; beyond that point, the deltoid continues to be the primary mover until the arm is fully raised. Therefore, the emphasis on this muscle's role in shoulder abduction makes it clear why this is the correct choice in the context of the question.