The actual sites of gas exchange within the lungs are
a) bronchioles.
b) pleural spaces.
c) terminal bronchioles.
d) interlobular septa.
e) alveoli.

The correct option is 'E': Alveoli.

Explanation:

The respiratory system consists of following passages:

When we inhale air from our nose it the air flows into nasal cavity from the nose where the nasal mucous acts as a filter of air.Next the air moves into pharynx  where the air is passed into a tube called as trachea. The air is prevented to enter esophagus by a flap called epiglottis.

The air from trachea is then directed to primary bronchi into left and the right lung and then to  secondary bronchi and then to tertiary bronchi, each acting as a branch of the preceding section. The tertiary bronchi further divide into bronchioles at whose ends sacs of thin membrane surrounded by blood capillaries are present. These sacs are termed as alveoli and it is at this place the transfer of gases occurs between air and the blood.

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