**Core Concept**
A vaccine trial in 1-year-old children requires a study design that assesses the efficacy of the vaccine in a population that has not been previously exposed to the disease. This necessitates a controlled study to evaluate the vaccine's ability to induce immunity and prevent disease.

**Why the Correct Answer is Right**
The correct answer is **C. Randomized Controlled Trial (RCT)**. In a RCT, participants are randomly assigned to either receive the vaccine (intervention group) or a placebo (control group). This design allows for the evaluation of the vaccine's efficacy and safety in a controlled manner. The study can be conducted in a double-blind fashion, where neither the participants nor the researchers know who is receiving the vaccine or placebo. This design is essential for establishing causality between the vaccine and the observed outcomes.

**Why Each Wrong Option is Incorrect**
**Option A:** A **Case-control study** is not suitable for evaluating vaccine efficacy in a population of 1-year-old children. This study design involves comparing individuals with a specific outcome (e.g., disease) with those without the outcome, but it does not establish causality.

**Option B:** A **Cohort study** can be used to evaluate vaccine efficacy, but it is not the best option for a study in 1-year-old children. In a cohort study, participants are followed over time to assess the development of outcomes, but it may take longer to establish efficacy.

**Option D:** An **Observational study** is not suitable for evaluating vaccine efficacy in a population of 1-year-old children. This study design involves observing participants without intervening, which can lead to biases and confounding variables.

**Clinical Pearl / High-Yield Fact**
When designing a vaccine trial, it is essential to consider the study population's demographics, disease prevalence, and potential confounding variables.

**Correct Answer:** C. Randomized Controlled Trial (RCT)