Site of gaseous exchange in lungs is

1. A

   Tracheoles

   Tracheoles are the fine air tubes of the insect tracheal system, not the human lung’s exchange surface. Naming tracheoles for mammalian lungs confuses arthropod respiration with ours. They are not the site asked.

2. B

   Bronchioles

   Bronchioles conduct air within the lung but have little surface for gas exchange compared with alveoli. They are airways, not the principal exchange membrane. The question asks the exchange site, not the last conducting tube.

3. C

   Pulmonary vein

   The pulmonary vein carries oxygenated blood from lungs to the left atrium; it is a vessel, not the air–blood interface. Exchange has already occurred before blood enters these veins. This option names transport, not the site.

4. D

   Alveoli

   (d) Official key: Alveoli are the thin-walled air sacs where oxygen and carbon dioxide diffuse between alveolar air and pulmonary capillary blood. Their huge combined surface and proximity to capillaries make them the site of gaseous exchange in lungs. Among tracheoles, bronchioles, pulmonary vein and alveoli, only alveoli are that site.

Summary. Official key is (d). In human lungs, gas exchange occurs across the alveolar–capillary membrane. Tracheoles belong to insects; bronchioles are mainly conducting; the pulmonary vein only carries already oxygenated blood. The keyed structure is therefore the alveoli.