What is the name of the muscle located under the arm that assists in lifting the arm and in breathing?

The correct choice is the Serratus anterior. This muscle is located on the side of the chest and plays a vital role in the movement of the shoulder and arm. Specifically, the Serratus anterior functions to lift the arm, especially when raising it above the shoulder. It is also involved in the breathing process by assisting with the expansion of the rib cage during inhalation.

The Serratus anterior is particularly important for scapular rotation, which is crucial for proper arm movement and stability. Its action allows for the upward rotation of the scapula, enabling the arm to be lifted overhead. This muscle is essential for activities such as pushing, throwing, or lifting, where the arms need to be elevated.

The other muscles mentioned, while also important, serve different primary functions. The Deltoid is primarily responsible for arm abduction and shoulder rotation, the Latissimus dorsi helps in arm extension and adduction as well as rotation, and the Pectoralis major mainly aids in shoulder flexion, adduction, and internal rotation. Therefore, the Serratus anterior is the most directly involved in lifting the arm and facilitating the breathing process in the context of the question.