**Core Concept**
The Sullivan index, also known as the Disability-Free Life Expectancy (DFLE), is a measure of the average number of years a person is expected to live free from disability. It takes into account both mortality and morbidity, providing a comprehensive picture of a population's health status.

**Why the Correct Answer is Right**
The Sullivan index is calculated by dividing the life expectancy at birth by the disability-free life expectancy at birth. This ratio gives an estimate of the proportion of life spent in a state of good health. The index is used to evaluate the effectiveness of healthcare systems and policies, as well as to identify areas where improvements are needed. The Sullivan index is particularly useful for comparing the health status of different populations and for tracking changes over time.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Sullivan index is not a measure of quality of life, but rather a specific measure of disability-free life expectancy.
**Option B:** This option is incorrect because the Sullivan index is not a type of life expectancy, but rather a ratio of life expectancy to disability-free life expectancy.

**Clinical Pearl / High-Yield Fact**
The Sullivan index is an important tool for policymakers and healthcare providers, as it helps to identify areas where interventions can be targeted to improve health outcomes. By reducing disability and increasing disability-free life expectancy, healthcare systems can improve the overall well-being of their populations.

**Correct Answer:** C.