**Core Concept**
The question pertains to the recommended vaccination schedule for children, specifically at the age of 5 years. This involves understanding the immunization protocol for preventing various infectious diseases in pediatric populations.

**Why the Correct Answer is Right**
At the age of 5 years, children are typically due for booster doses of several vaccines to maintain immunity and protect against vaccine-preventable diseases. The most relevant vaccine at this age is the MMR (Measles, Mumps, and Rubella) vaccine, which is administered as a second dose to ensure long-term protection against these viral infections. According to guidelines, the MMR vaccine is given at 12-15 months and a second dose at 4-6 years of age. This second dose of MMR vaccine is crucial for maintaining herd immunity and preventing outbreaks of these diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Hepatitis B vaccine series is typically completed by 6-12 months of age, and booster doses are not recommended at age 5.
**Option B:** This option is incorrect because the Varicella (chickenpox) vaccine is usually given to children at 12-15 months and a second dose at 4-6 years, similar to the MMR vaccine. However, the exact timing might vary depending on the country's guidelines.
**Option C:** This option is incorrect because the Hib (Haemophilus influenzae type b) vaccine series is completed by 12-15 months of age, and booster doses are not recommended at age 5.

**Clinical Pearl / High-Yield Fact**
It's essential for pediatricians and primary care physicians to ensure timely administration of booster doses to maintain immunity against vaccine-preventable diseases. A well-coordinated vaccination schedule is crucial for preventing outbreaks and protecting vulnerable populations.

**Correct Answer: B. Varicella vaccine.**