Answer:
d. alveoli
Step-by-step explanation:
Alveoli -
It is an important and crucial part of the respiratory system , the main function of alveoli is the exchange of gases ,
The structure of Alveoli is a balloon shaped air sacs , which are present in the terminating end of the bronchial branches .
The thickness of alveoli is only one cell thick , whihc allows the exchange of gases .