Tachypnea can be caused by which of the following factors?

Tachypnea, or an increased respiratory rate, can occur due to various physiological and pathological conditions. Hypoxia, which is a deficiency in the amount of oxygen reaching the tissues, can lead to an increased respiratory rate as the body attempts to compensate for the lack of oxygen. In response to low oxygen levels, the respiratory center in the brain signals the body to breathe more rapidly to enhance oxygen intake and improve gas exchange, thereby increasing the respiratory rate.

Other options are not typically associated with causing tachypnea. For instance, sleep often leads to a decrease in respiratory rate, and atrophy refers to the wasting of tissue or an organ, which wouldn't directly cause an increased respiratory rate. Similarly, exhalation is a normal component of the breathing cycle and does not inherently trigger an increase in respiratory rate. Thus, hypoxia is a clear and direct cause of tachypnea, making it the correct answer.