**Core Concept**
The Framingham Heart Study is a long-term, ongoing cardiovascular cohort study on residents of the town of Framingham, Massachusetts, that has provided valuable insights into the risk factors and relative importance of various risk factors for coronary artery disease (CAD) and other cardiovascular conditions.

**Why the Correct Answer is Right**
The Framingham Heart Study, initiated in 1948, has been instrumental in identifying key risk factors for CAD, including age, sex, total cholesterol, high-density lipoprotein (HDL) cholesterol, systolic blood pressure, smoking status, and diabetes mellitus. These factors are used to calculate the 10-year risk of developing CAD, as outlined in the Framingham Heart Study risk equation. The study has also provided data on the relative importance of these risk factors, with age, total cholesterol, and systolic blood pressure being the most significant predictors of CAD risk.

**Why Each Wrong Option is Incorrect**
**Option A:** The National Health and Nutrition Examination Survey (NHANES) is a program of studies designed to assess the health and nutritional status of adults and children in the United States. While NHANES provides valuable data on various health indicators, it is not specifically focused on coronary risk factors.
**Option B:** The Multiple Risk Factor Intervention Trial (MRFIT) was a study that examined the effects of treating high blood pressure and high cholesterol on the risk of heart disease in middle-aged men. While MRFIT provided important data on the impact of these risk factors, it did not specifically examine the relative importance of various risk factors for CAD.
**Option C:** The Nurses' Health Study (NHS) is a long-term prospective cohort study of female nurses that has examined various risk factors for cardiovascular disease, including diet, physical activity, and hormone replacement therapy. While NHS has provided valuable insights into the relationship between lifestyle factors and CAD risk, it is not specifically focused on the relative importance of various risk factors.

**Clinical Pearl / High-Yield Fact**
The Framingham Heart Study risk equation is a widely used tool for estimating CAD risk, and it is essential to understand the relative importance of various risk factors in this equation to provide effective cardiovascular risk assessment and management.

**Correct Answer:** C. The Framingham Heart Study has given coronary risk factors and their relative importance.