Final answer:
The right splenius capitis muscle is antagonistic to the unilateral action of the left sternocleidomastoid.
Step-by-step explanation:
The muscle that is antagonistic to the unilateral action of the left sternocleidomastoid is the right splenius capitis. The sternocleidomastoid muscle is responsible for laterally flexing and rotating the head, while the right splenius capitis muscle performs the opposite action. When the left sternocleidomastoid contracts unilaterally, it causes the head to rotate towards the right side. The right splenius capitis muscle contracts to rotate the head towards the left side, thereby acting antagonistically to the unilateral action of the left sternocleidomastoid.