**Core Concept:**
The Sullivan index is a measure of the prevalence of vision impairment and blindness in a population based on the World Health Organization (WHO) classification. It is calculated using the following formula:

Number of visually impaired individuals (15/20 ≤ visual acuity < 3/60) + Number of blind individuals (visual acuity ≤ 3/60) / Total number of examined individuals × 100 **Why the Correct Answer is Right:** The correct answer is referring to the Sullivan index formula, which is used to calculate the prevalence of vision impairment and blindness in a population. The formula takes into account both visually impaired (15/20 ≤ visual acuity < 3/60) and blind individuals (visual acuity ≤ 3/60) and divides the sum by the total number of examined individuals. This approach provides a comprehensive estimation of visual impairment and blindness in a population. **Why Each Wrong Option is Incorrect:** A. This option is incorrect because it does not represent a component of the Sullivan index formula. B. This option is incorrect because it only considers visually impaired individuals with visual acuity between 15/20 and 3/60, not blind individuals with visual acuity ≤ 3/60. C. This option is incorrect because it only considers visually impaired individuals, excluding blind individuals. D. This option is incorrect because it does not include the total number of examined individuals in the calculation, thus not giving a precise estimation of the prevalence in the population. **Clinical Pearl:** The Sullivan index is essential for public health planning and assessment of the effectiveness of interventions aimed at preventing and treating visual impairment and blindness. By accurately estimating the prevalence of visual impairment and blindness, healthcare professionals and policymakers can prioritize resources, develop targeted interventions, and evaluate the success of prevention and treatment programs.