Which structures forms a complete ring around the airway?

Cricoid cartilage.

Explanation:

Cricoid cartilage is a type of hyaline cartilage that surrond the trachea which is found near the center of the neck. It consist of strong connective tissue that comprises of the back of larynx. It overlapp the tracheal ring and the most caudal of the trachea cartilage that form complete ring around the air.

It function to provide attachment to the cricoid muscles, cartilage and ligaments that is involved in opening and closing of air ways and when speech is produce.

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