The structural and functional unit of the lungs is:

Explanation

Option A: Lungs are the organs of ventilation in terrestrial animals. In humans, nostrils lead to the trachea or windpipe through the nasopharynx. The trachea is a straight tube extending up to the mid-thoracic cavity, which divides at the level of the 5th thoracic vertebra into a right and left primary bronchi. Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi and bronchioles ending up in very thin terminal bronchioles. The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. Each terminal bronchiole gives rise to several very thin, irregularly walled and vascularised bag-like structures Option B: Trachea Bronchiole, and Bronchus are not the correct answers because they are all part of the larger airway system that leads to the alveoli. Option C: Trachea, Bronchiole, and Bronchus are not the correct answers because they are all part of the larger airway system that leads to the alveoli. Option D: Trachea, Bronchiole, and Bronchus are not the correct answers because they are all part of the larger airway system that leads to the alveoli.