Site of gaseous exchange in lungs is

The correct answer is D) Alveoli. Gas exchange happens across the alveolar walls, which are a single cell thick, and the surrounding capillaries are close enough for rapid diffusion. Oxygen moves from the air into the blood and carbon dioxide moves the other way. The very large surface area, roughly 70 to 80 square metres in an adult, makes this efficient.

• Tracheoles are the fine terminal tubes of the insect respiratory system, not the human lung. Aspirants pick this because of the name.
• Bronchioles belong to the conducting zone. They carry air but do not exchange gases. Only respiratory bronchioles have a few alveoli budded on their walls, and the main exchange site is still the alveoli.
• Pulmonary vein carries oxygenated blood from the lungs to the left atrium. It is a transport vessel, and exchange has already happened before blood enters it.

The core concept is the structure of the respiratory membrane. It links to basic human physiology and to Mains topics on air pollution and disease, such as emphysema, where alveolar walls are destroyed and gas exchange falls.

Trap: choosing bronchioles because they look like the last airways in the lung, or tracheoles because of the shared 'trache' root.