............. is the only complete ring of the cartilage around the trachea.

The correct option is B Cricoid cartilage
Cricoid cartilage is the ring or complete circular cartilage in the neck. This is the cartilage which is the complete cartilage of the trachea. This is situated to the inferior of the thyroid cartilage. This also forms the back portion of the larynx. This cartilage serves as the point of attachment of the muscles, ligaments and other types of cartilage which are involved in the speech movement.

Thus, the correct answer is option B.