Answer: b. cricoid cartilage.
Step-by-step explanation:
The larynx cavity extends from the triangle shaped inlet that is called as the epiglottis which opens to the circular outlet that is called as circoid cartilage at the lower border. The circoid cartilage forms continuity with that of the lumen of the trachea. The mucous membrane which provides the lining to the larynx forms two pairs of the lateral folds that opens inward inside the cavity.