**Core Concept**
Sullivan's Index is a measure of the prevalence of disability in a population, focusing on the proportion of individuals who are unable to work due to disability. It is a widely used indicator in public health to assess the burden of disease and disability in a community. Sullivan's Index is calculated as the percentage of a population that is expected to be free of disability.

**Why the Correct Answer is Right**
Sullivan's Index is named after John W. Sullivan, who developed it in the 1950s. The index takes into account the age-specific prevalence of disability and the life expectancy of a population. It is calculated by subtracting the years of expected life with disability from the total years of expected life, and then dividing by the total years of expected life. This index is useful for comparing the health status of different populations and for evaluating the effectiveness of health interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Sullivan's Index is not a measure of mortality rate. It focuses on the prevalence of disability rather than the number of deaths.
**Option B:** This option is incorrect because the Disability-Adjusted Life Years (DALYs) measure is a different indicator that takes into account both the years of life lost due to premature death and the years lived with disability.
**Option C:** This option is incorrect because the Global Burden of Disease (GBD) study is a comprehensive assessment of the health status of populations, but it is not a specific measure like Sullivan's Index.

**Clinical Pearl / High-Yield Fact**
Sullivan's Index is a useful tool for public health officials to assess the burden of disease and disability in a community, and to evaluate the effectiveness of health interventions. It can be used to compare the health status of different populations and to identify areas where health interventions may be needed.

**Correct Answer:** D.