**Core Concept:** Person-years is a statistical term used to measure the total work experience contributed by a group of individuals in terms of person-years. In this context, person-years refers to the total labor input or employment duration of individuals.

**Why the Correct Answer is Right:** To calculate the total person-years, we simply multiply the number of individuals (25 persons) by the duration of employment (30 years). In this case, the correct answer is **Option D**.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** Multiplying 25 persons by 30 years would give the total number of people, not person-years. This is incorrect because it does not consider the combined work experience of all individuals.

B. **Option B:** Multiplying 25 by 30 does not account for the individual durations of employment.

C. **Option C:** Multiplying 2500 (25 x 30) does not differentiate between the individual employment durations, making it incorrect as well.

**Clinical Pearl / High-Yield Fact:** Understanding person-years helps in epidemiological studies, particularly when assessing the contribution of risk factors on the development of diseases or outcomes. It is often used in cancer incidence or mortality studies to estimate the number of years at risk.

**Correct Answer:** AIIMS (All India Institute of Medical Sciences) is a group of medical institutes in India, which is not relevant to the calculation of person-years mentioned in the question. Therefore, the correct answer is **Option D**.