**Core Concept**
The production of certain vaccines involves the use of embryonated hen's eggs as a substrate for the growth of the virus. This method is used for vaccines that require the replication of the virus in a living host to produce a high yield of viral particles.

**Why the Correct Answer is Right**
Rabies vaccine is one of the classical examples of a vaccine prepared using embryonated hen's eggs. The rabies virus is grown in the eggs, which provides a suitable environment for the virus to replicate and produce high titers of the virus. The inactivated virus is then harvested and used as the vaccine. This method is particularly useful for producing vaccines that require a high yield of viral particles, such as the rabies virus.

**Why Each Wrong Option is Incorrect**
**Option A:** Measles vaccine is typically produced using a combination of chick embryo cell cultures and human diploid cells, not embryonated hen's eggs.
**Option C:** Rubella vaccine is also produced using human diploid cell cultures, not embryonated hen's eggs.
**Option D:** Varicella vaccine is produced using human diploid cell cultures, not embryonated hen's eggs.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the use of embryonated hen's eggs in vaccine production has some limitations, such as the potential for contamination with other pathogens and the risk of allergic reactions to egg proteins in the vaccine. However, the benefits of this method, including the high yield of viral particles and the cost-effectiveness, have made it a widely used approach in vaccine production.

✓ Correct Answer: B. Rabies