Inhalation is a physical process by which a volume of air is taken into the thoracic cavity. This is the first process of respiration in man. The other steps include external, internal and cellular respiration.
For a large volume of air to enter thoracic cavity, it is imperative that the volume of thoracic cavity must increase.
A. Contraction of diaphragm does not decrease the volume of chest cavity.
C. Recoil of lungs due to the contraction of muscles could enable the thorax to decrease in size but this is not a process directly involved with inhalation.
D. Recoil of lungs due to the contraction of muscles does not enable the thorax to increase in size.
Therefore, options A, C & D are incorrect.
Contraction of the diaphragm enlarges the chest cavity (B), which enables us to take in air. In humans, diaphragm is a thin sheet of muscles bordering the bottom of the thoracic cavity. Enlargement of thoracic cavity by diaphragm distension causes increase in volume and a relatively negative pressure within the thoracic cavity, and this in turn, allows air to enter the lungs.
Therefore, option (B) is correct.