Name the muscle primarily responsible for neck flexion.

The sternocleidomastoid muscle is primarily responsible for neck flexion due to its anatomical position and function. This muscle originates from the manubrium of the sternum and the medial portion of the clavicle and inserts into the mastoid process of the temporal bone of the skull. When both sides of the sternocleidomastoid contract simultaneously, they pull the head forward, resulting in neck flexion.

Moreover, the muscle plays a crucial role in the rotational movements of the head when one side contracts, further emphasizing its importance in neck mobility and stability. This makes it the primary muscle involved in neck flexion compared to the other options listed, which may play supportive or secondary roles in neck movement but are not the main drivers of flexion.