According to central place theory, the reason cardiologists are more rare than general practice doctors is primarily related to the concept of "threshold" and "range."
The correct response would be: "A high level of population density is required for a cardiology clinic’s threshold."
This implies that cardiology services need a larger population to support their specialized services compared to general practice, which can operate effectively with a smaller population base. Cardiologists often require a higher threshold population because their services are specialized and not needed as frequently, thus making them less ubiquitous in less populated areas.