**Core Concept**
The underlying principle being tested is the establishment of causal association in the aetiology of a disease, which involves various criteria to determine whether a particular factor is the cause of a disease. **Causal association** is crucial in epidemiology to understand the relationship between risk factors and diseases.

**Why the Correct Answer is Right**
Although the options are missing, the most difficult criterion to establish is often considered to be **Temporal Relationship**, which requires that the cause precedes the effect in time. However, without the options provided, we can infer that other challenging criteria might include **Specificity**, which is difficult to establish when a single cause has multiple effects or when a single effect has multiple causes.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option, we cannot directly address why it is incorrect, but we can say that if it were a criterion like **Strength of Association**, it would be incorrect because while a strong association is suggestive of causality, it is not the most difficult criterion to establish.
**Option B:** Similarly, if this option were **Dose-Response Relationship**, it would be incorrect because, although a dose-response relationship supports causality, it may not always be present or easily demonstrable.
**Option C:** If this option were **Consistency**, it would be incorrect because consistency across different studies is important for establishing causality, but it is not typically the most difficult criterion to meet.
**Option D:** Without knowing the specific option, we cannot directly address its incorrectness, but each of the criteria for causal association has its challenges.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that establishing causal association involves several criteria, including **Temporal Relationship**, **Strength of Association**, **Dose-Response Relationship**, **Consistency**, **Specificity**, **Plausibility**, **Coherence**, **Experiment**, and **Analogy**. These criteria help in determining whether an observed association is causal.

**Correct Answer:** D. Temporal Relationship.