**Core Concept**
Observational studies are used to investigate the relationship between risk factors and disease outcomes. In epidemiology, study design plays a crucial role in establishing causality. A weaker study design may not provide reliable evidence for causation.

**Why the Correct Answer is Right**
Option C: **Case Series** is the weakest study design among the given options. This is because a case series only describes the characteristics of a group of patients with a particular condition, without comparing them to a control group. As a result, it is unable to establish a cause-and-effect relationship between the risk factor and the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** **Cross-sectional study** is a weaker study design than cohort studies or randomized controlled trials, but it still allows for the comparison of exposed and unexposed groups, making it stronger than case series.
**Option B:** **Case-control study** is a retrospective study design that can establish temporal relationships between risk factors and disease, making it a stronger study design than case series.

**Clinical Pearl / High-Yield Fact**
When evaluating observational studies, consider the study design, sample size, and potential biases that may affect the results. A strong study design is essential for establishing causality and informing clinical practice.

**Correct Answer:** C. Case Series.