Final answer:
The Superior oblique muscle (E) is responsible for rotating the eye diagonally downward towards the middle of the head by medially rotating the eye, a movement referred to as intorsion. Therefore, the correct option is E.
Step-by-step explanation:
The muscle that helps in moving the eye diagonally downward towards the middle of the head is the Superior oblique muscle (E). This muscle originates at the posterior orbit and its tendon goes through a pulley-like structure known as the trochlea. The contraction of the superior oblique muscle results in the rotation of the eye medially, which is referred to as intorsion. It is also responsible for movements that are part of focusing the eyes on objects that are close to the face. On the other hand, the Inferior oblique muscle (C) originates from the floor of the orbit and inserts into the inferolateral surface of the eye. It is responsible for the lateral rotation of the eye. When the eye moves downwards, the inferior oblique balances out the movement of the inferior rectus, which pulls the eye at an angle rather than straight down.