What is the main action of the deltoid muscle?
a. adduction of the arm at the shoulder joint.
b. retraction of the scapula.
c. depression of the scapula.
d. abduction of the arm at the shoulder joint.

Option (d).

Explanation:

Deltoid muscle forms the rounded contour of the shoulder. This muscle is also known as common shoulder muscle. This muscle is triangle in shape.

Deltoid muscle acts as a prime abductor of the shoulder. This muscle causes abduction of the arms at the shoulder joint of humans. This muscle also involved in the arm extension.

Thus, the correct answer is option (d).