**Core Concept**
The question is testing the student's knowledge of the recommended vaccination schedule for infants and toddlers. The underlying principle is the timing and sequence of administration of various vaccines to prevent vaccine-preventable diseases.

**Why the Correct Answer is Right**
The correct answer is a vaccine that can be administered at 18 months of age. The measles, mumps, and rubella (MMR) vaccine is typically given in two doses, with the first dose at 12-15 months and the second dose at 4-6 years of age. However, some schedules may allow for the first dose to be administered as early as 12 months or as late as 18 months. The MMR vaccine is crucial for preventing measles, mumps, and rubella, which can cause serious complications, including encephalitis, deafness, and birth defects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a vaccine that is typically administered at 18 months of age. The first dose of the varicella (chickenpox) vaccine is usually given at 12-15 months, and the second dose is given 3-5 months later.
**Option B:** This option is incorrect because it is not a vaccine that is typically administered at 18 months of age. The first dose of the Haemophilus influenzae type b (Hib) vaccine is usually given at 2 months, and booster doses are given at 4 months and 12-15 months.
**Option C:** This option is incorrect because it is not a vaccine that is typically administered at 18 months of age. The first dose of the Hepatitis A vaccine is usually given at 12 months, and a second dose is given 6-12 months later.

**Clinical Pearl / High-Yield Fact**
Remember that the vaccination schedule may vary depending on the country, region, or individual patient's risk factors. Always consult the recommended schedule from reputable sources, such as the Centers for Disease Control and Prevention (CDC) or the World Health Organization (WHO).

**Correct Answer:** C.