Which of the following is NOT a cause of tachypnea?

Tachypnea, defined as an increased respiratory rate, can be influenced by a variety of physiological and pathological factors. In this case, the option indicating "calmness" is the correct choice because calmness is generally associated with a lower respiratory rate rather than an increased one. An individual who is calm tends to have a relaxed state, leading to a more stable and slower breathing pattern.

In contrast, hypoxia typically results in an increased respiratory rate as the body attempts to bring in more oxygen to compensate for lower oxygen levels in the blood. Fever raises the metabolic rate and can lead to tachypnea as the body tries to cool itself down and manage increased demands for oxygen. CNS (central nervous system) problems can also lead to alterations in breathing patterns, including hyperventilation and increased respiratory rates due to various conditions affecting the brain.

Therefore, calmness stands out as the only option that does not contribute to the increase in respiratory rate, making it the correct choice for this question regarding causes of tachypnea.