What is one of the actions of the subscapularis muscle?

The subscapularis muscle is primarily responsible for rotating the shoulder medially, which is also known as internal rotation. It is one of the four rotator cuff muscles and plays a critical role in shoulder stability and function. Additionally, the subscapularis can assist with extending the shoulder when the arm is in a certain position, such as when the arm is flexed at the elbow and then extended in a movement pattern. This is why the choice mentioning the extension of the shoulder and medial rotation is correct; it accurately reflects the muscle's primary action of medial rotation along with its contributory role in shoulder extension.

All other options are less accurate because they either describe actions that are not associated with the subscapularis or involve movements that pertain to different muscles or joints.