Which prefix is related to the heart?

The prefix that relates to the heart is "cardi." This prefix is derived from the Greek word "kardia," which means heart. In medical terminology, "cardi" is commonly used to form words associated with the heart, such as "cardiology" (the study of the heart) and "cardiovascular" (pertaining to the heart and blood vessels). Understanding this prefix is essential for grasping various terms used in healthcare and medicine that involve heart-related functions or conditions.

The other prefixes do not pertain to the heart: "cephal" refers to the head, "sub" means under or below, and "tetra" denotes the number four. Thus, only "cardi" directly connects to the heart, making it the correct choice in this context.