Final answer:
The deltoid muscle is responsible for abducting and extending the arm at the shoulder.
Step-by-step explanation:
The muscle that acts to abduct and extend the arm at the shoulder is the deltoid. The deltoid is a large, triangular-shaped muscle that covers the shoulder joint. It is involved in various movements. When the deltoid muscle contracts, it not only abducts the arm, moving it away from the midline, but it also helps in flexing and medial rotation as well as extension and lateral rotation of the arm. It is also known as the 'common shoulder muscle', particularly in other animals such as the domestic cat.