The prefix "cardio-" is primarily used in which field?

The prefix "cardio-" is derived from the Greek word "kardia," meaning "heart." It is primarily used in the field of cardiology, which focuses on the study and treatment of disorders related to the heart and blood vessels. This prefix appears in a variety of terms related to heart health, such as "cardiovascular" (pertaining to the heart and blood circulation) and "cardiomyopathy" (a disease of the heart muscle).

In contrast, the other fields mentioned do not focus on the heart or its functions. Geology pertains to the Earth and its processes, biology concerns the study of living organisms and life processes, and psychology centers around the mind and behavior. Therefore, the relevance of "cardio-" is specifically tied to cardiology, making it the correct association.