**Core Concept:** Measles is a highly contagious viral disease that primarily affects children. Vaccines, such as the measles vaccine, are used to prevent infectious diseases like measles by stimulating the immune system to produce an immune response without causing the actual disease. The World Health Organization (WHO) recommends administering the measles vaccine in two doses:

**Why the Correct Answer is Right:** According to WHO guidelines, the first dose of the measles vaccine is usually given at 12-15 months of age (Option D). This early administration ensures that the child develops immunity before being exposed to the virus in their environment.

**Why Each Wrong Option is Incorrect:** Option A (6 months) is too early as the immune system is not fully developed at this age. Option B (18 months) is too late as the child may be exposed to the virus in the meantime, increasing the risk of infection. Option C (9 months) is also too early, and the child may not develop a strong immune response due to the vaccine being given before they are old enough to understand and cooperate during subsequent doses.

**Clinical Pearl / High-Yield Fact:** The second dose of the measles vaccine is typically administered between 12 and 15 months of age (Option D). This age range ensures that the child develops a robust immune response, providing long-lasting protection against measles.

**Why the Correct Answer is Right:** The second dose is given between 12 and 15 months to reinforce the initial immune response and ensure long-lasting immunity, as well as to protect against breakthrough infections in vaccinated individuals who are exposed to the virus.

**Why Each Wrong Option is Incorrect:** Option A (10 months) is too early as the immune response may not be robust enough to provide long-lasting protection. Option B (18 months) is too late, increasing the risk of exposure to measles virus during this period. Option C (15 months) is also too late, as it may not allow sufficient time for a robust immune response development before the child becomes more mobile and exposed to the virus.

**Correct Answer:** D. 12-15 months.