## Core Concept
Randomization in clinical trials is a crucial methodological element that aims to minimize bias and ensure that the groups being compared are similar in terms of both observed and unobserved factors. This process helps in establishing cause-and-effect relationships between the intervention and the outcome. By randomly assigning participants to treatment groups, researchers can control for confounding variables.

## Why the Correct Answer is Right
The main purpose of randomization is to ensure that the groups being compared are similar in all aspects, both known and unknown, except for the treatment they receive. This similarity helps in isolating the effect of the treatment from other influencing factors, thereby providing a more accurate estimate of the treatment effect. Randomization works on the principle that as the sample size increases, the groups become more similar in terms of baseline characteristics.

## Why Each Wrong Option is Incorrect
- **Option A:** While ensuring that the groups have an equal number of participants is a potential benefit of randomization, it is not the main purpose. Randomization does not guarantee equal numbers in each group, especially in smaller trials, but it does ensure that any differences are due to chance.
- **Option B:** Although minimizing bias in the selection of participants is a critical aspect of study design, and randomization helps achieve this, stating it as ensuring that "the groups are similar in terms of observed factors" might overlook the importance of unobserved factors. However, this option might seem plausible but does not fully capture the essence of why randomization is primarily used.
- **Option D:** Ensuring that the groups have similar numbers of participants is not the primary goal; rather, it's about making the groups comparable in terms of baseline characteristics.

## Clinical Pearl / High-Yield Fact
A key point to remember is that randomization helps in reducing confounding variables and ensures that the only difference between the groups is the intervention being tested. This concept is critical in understanding the validity and reliability of clinical trial results.

## Correct Answer: C