**Core Concept**
Observational study designs are used to evaluate the association between risk factors and diseases. These designs are categorized based on their ability to establish causality, with stronger designs providing more convincing evidence of a causal relationship.

**Why the Correct Answer is Right**
The weakest study design among the options is the **Cross-Sectional Study**. This design involves analyzing a defined population at a single point in time to determine the prevalence of a disease or exposure and its associated risk factors. However, cross-sectional studies are prone to **reverse causality**, where the outcome is a result of the exposure, rather than the exposure causing the outcome. Additionally, this design does not allow for the temporal relationship between the exposure and outcome to be established.

**Why Each Wrong Option is Incorrect**
* **Option A: Case-Control Study**. While case-control studies can also be prone to biases, such as recall bias and selection bias, they can still establish an association between the exposure and outcome by comparing individuals with the disease (cases) to those without the disease (controls).
* **Option B: Cohort Study**. Cohort studies are considered a stronger design than cross-sectional studies as they can establish a temporal relationship between the exposure and outcome, allowing for the assessment of incidence rates and the potential for causal inference.
* **Option C: Randomized Controlled Trial (RCT)**. RCTs are considered the gold standard in experimental design as they involve random allocation of participants to treatment or control groups, minimizing confounding variables and allowing for causal inference.

**Clinical Pearl / High-Yield Fact**
When evaluating study designs, remember that **temporality** is crucial in establishing a causal relationship between risk factors and diseases. Stronger study designs, such as cohort studies and RCTs, allow for the establishment of a temporal relationship between the exposure and outcome.

**Correct Answer:** D. Cross-Sectional Study.