The circular muscle of the mouth

a. Frontalis

b. Orbicularis oris

Orbicularis oris

Explanation:

Orbicularis oris muscle is present in the lips. These muscles may lie horizontally or vertically with other muscles.

Orbicularis oris is the circular muscle that encircles the mouth. This muscle is also known as kissing muscle. Orbicularis oris muscle originates from the maxillae and mandible and innervated by the facial nerve. This muscle inserts in the lips margin.

Thus, the correct answer is option (B).