**Core Concept**
Randomization in clinical trials is a fundamental aspect of ensuring the internal validity of the study. It involves the process of assigning participants to different treatment groups in a random manner to minimize bias and ensure that the groups are comparable in terms of known and unknown confounding variables.

**Why the Correct Answer is Right**
The major purpose of randomization is to eliminate selection bias and ensure that the groups are comparable in terms of both observed and unobserved variables. This is achieved by using a randomization algorithm, such as a random number generator, to assign participants to treatment groups. Randomization helps to distribute both known and unknown confounding variables evenly across the groups, allowing for a fair comparison of the treatment effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** Randomization is not primarily used to eliminate selection bias, although it does play a role in this process. It is more accurately described as a way to ensure that the groups are comparable in terms of both observed and unobserved variables.
* **Option B:** Randomization is not used to reduce the sample size of the study. In fact, randomization can sometimes lead to slightly larger sample sizes due to the need to account for dropouts and missing data.
* **Option C:** Randomization is not a substitute for controlling for confounding variables through statistical analysis. While randomization can help to minimize the impact of confounding variables, it is not a guarantee that all confounding variables will be eliminated.

**Clinical Pearl / High-Yield Fact**
When interpreting the results of a clinical trial, it is essential to consider whether randomization was used and whether the groups were comparable in terms of both observed and unobserved variables. This can help to increase the confidence in the validity of the study findings.

**Correct Answer: C.**