**Core Concept**
Sullivan's 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 is an important indicator of the health and well-being of a population, taking into account both mortality and morbidity rates.

**Why the Correct Answer is Right**
Sullivan's index is calculated by dividing the Disability-Free Life Expectancy (DFLE) by the Life Expectancy (LE) at birth. This ratio provides a more comprehensive understanding of the health status of a population, beyond just mortality rates. The DFLE is calculated by subtracting the years lived with disability (YLD) from the Life Expectancy (LE) at birth. This measure is useful for evaluating the effectiveness of healthcare interventions and policies aimed at improving population health.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Sullivan's index specifically focuses on disability-free life expectancy, not overall life expectancy.
**Option B:** This option is incorrect because Sullivan's index is not a direct measure of quality-adjusted life years (QALYs), although both metrics are related to health outcomes.
**Option C:** This option is incorrect because the Disability-Adjusted Life Years (DALYs) measure combines years of life lost due to premature mortality and years lived with disability, whereas Sullivan's index focuses solely on disability-free life expectancy.

**Clinical Pearl / High-Yield Fact**
Sullivan's index is a useful tool for policymakers and healthcare professionals to evaluate the impact of healthcare interventions on population health, beyond just mortality rates. It highlights the importance of addressing both mortality and morbidity to improve overall health outcomes.

**Correct Answer: D. Disability-free life expectancy.