**Core Concept**
Randomization is a fundamental aspect of clinical trials, ensuring that the treatment groups are comparable in terms of known and unknown confounding variables. The randomization unit refers to the entity to which the randomization process is applied, which is crucial in determining the internal validity of the trial.

**Why the Correct Answer is Right**
In most intervention trials, the preferred randomization unit is an individual, as it allows for the comparison of treatment effects at the individual level. This approach is particularly useful in assessing the efficacy and safety of interventions. Randomizing at the individual level also enables the assessment of the effect of the intervention on specific outcomes, such as disease progression or symptom severity.

**Why Each Wrong Option is Incorrect**
**Option A:**
Randomizing at the cluster level, such as by clinic or hospital, is often used in trials where the intervention is applied at the group level, or where it is not feasible to randomize at the individual level. However, this approach can lead to issues with statistical analysis and may not accurately reflect the true effect of the intervention.

**Option B:**
Randomizing at the family or household level is sometimes used in trials where the intervention has a significant impact on family members or where the outcome of interest is a family-level outcome. However, this approach can be challenging to implement and may not be suitable for all types of interventions.

**Option C:**
Randomizing at the organ or tissue level is not typically used in intervention trials, as it is often not feasible to apply interventions at this level. However, this approach may be used in certain types of experimental studies, such as those involving gene therapy or tissue engineering.

**Clinical Pearl / High-Yield Fact**
When designing an intervention trial, it is essential to carefully consider the randomization unit to ensure that the trial is adequately powered and that the results accurately reflect the effect of the intervention.

**Correct Answer:** D