During inhalation:
a. the diaphragm relaxes.
b. the diaphragm and rib muscles contract.
c. oxygen molecules move into the lungs, and carbon dioxide molecules move out of the lungs.
d. the volume of the thoracic cavity decreases.
e. air moves up the trachea.

The diaphragm and rib muscles contract. (Ans B).

Explanation:

Breathing in is the process which is also known as inhalation. Inhalation is a process where taking air and sometimes some other substances, and other gases enter the lungs.

When we inhale or breathe in our diaphragm moves downward and contract. This helps to increase the space in our chest cavity and lungs expand into it. The muscles which are present between our ribs also help to enlarge the cavity of the chest. When we breathe they contract to pull our rib cage both outward and upward directions.