Surfactant to prevent the alveoli from collapsing by

The correct answer will be- by lowering the surface tension at air/liquid transition.

Explanation:

Pulmonary surfactant is the mixture of proteins and lipids produced by the type II epithelial cells.  

The surfactant is selected into the alveolar sacs where the surfactant reduces the surface tension of the air/liquid interface to 1 mN/m. The lipid content of the surfactant called or DPPC forms a surface film which lowers the surface tension.

This power surface tension reduces the work of breathing and thus prevent the alveolar sac from rupture or collapse.

Thus, by lowering the surface tension at air/liquid transition is the correct answer.