**Core Concept**
Measles vaccine administration within a specified timeframe after exposure is a critical public health strategy to prevent outbreaks and control the spread of the disease. This approach relies on the immune system's ability to generate a protective response against the measles virus within a short period.

**Why the Correct Answer is Right**
The correct answer is based on the concept of post-exposure prophylaxis (PEP) with measles vaccine. Studies have shown that administering the measles vaccine within 72 hours (3 days) after exposure can significantly reduce the risk of developing measles. This is because the vaccine can still induce a robust immune response, even if the individual has already been infected. The measles virus has a incubation period of about 10-14 days, and administering the vaccine within 3 days of exposure can prevent the virus from establishing a productive infection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the optimal timeframe for post-exposure prophylaxis with measles vaccine.
**Option B:** This option is incorrect because it suggests that the vaccine can be administered too late, allowing the virus to establish an infection.
**Option C:** This option is incorrect because it implies that the vaccine is ineffective if administered after a certain period.

**Clinical Pearl / High-Yield Fact**
It's essential to note that post-exposure prophylaxis with measles vaccine is most effective when administered within 72 hours of exposure. After this timeframe, the vaccine may still be effective but is less likely to prevent the development of measles.

**Correct Answer: A. 3 days**