During which phase of respiration can the breath be held the longest?

Inhalation.

Explanation:

Respiration may be defined as the process of exchange of gases between the body tissues, lungs and the outside environment. The oxygen is inhaled and carbon dioxide is exhaled out from the body.

Two phase of respiration are inhalation and exhalation. Inhalation is the the movement of oxygen inside the lungs and body tissues. The diaphragm contracts and moves downward which increases the space in chest cavity and the breathe can be held longest during this phase.

Thus, the answer is inhalation.