Which one of the following word parts means "air"?

The word part that means "air" is "pneum-." This root is derived from the Greek word "pneuma," which translates to "breath" or "air." In medical terminology, it is commonly found in words related to the respiratory system, such as "pneumonia" (inflammation of the lung tissue typically caused by an infection) and "pneumothorax" (the presence of air in the pleural space, which can cause lung collapse). Understanding the significance of this root helps in deciphering various medical terms associated with breathing and respiratory functions.

The other options do not pertain to "air" in their meanings. For instance, "my-" often refers to muscle or relates to the root "myo," as seen in terms like "myocardium" (heart muscle). "Retro-" indicates behind or backward in time, often used in terms like "retrograde." "Enter-" pertains to the intestines, used in words like "enteritis" (inflammation of the intestine). Thus, "pneum-" is the only prefix that correctly signifies "air."