**Core Concept**
The Sullivan index is a measure used to assess the health expectancy of a population. It takes into account the average number of years a person is expected to live in good health, considering both mortality and disability.

**Why the Correct Answer is Right**
The Sullivan index is a composite measure that combines life expectancy at birth with the expected number of years lived in good health. It is calculated by subtracting the years lived with disability from the total life expectancy. This index is useful in public health and epidemiology to evaluate the impact of various factors on the health status of a population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not correct as the Sullivan index is not a measure of economic productivity.
**Option B:** This option is also incorrect as the Sullivan index is not a measure of disease prevalence.
**Option C:** This option is not correct as the Sullivan index is not a measure of healthcare utilization.

**Clinical Pearl / High-Yield Fact**
The Sullivan index is an important tool in public health and epidemiology, as it helps to identify areas where interventions can be targeted to improve health expectancy and reduce years lived with disability.

**Correct Answer: D. Life expectancy in good health.**