**Core Concept**
Randomization in clinical trials is a method of assigning participants to treatment or control groups to minimize bias and ensure that the groups are comparable in terms of known and unknown confounding variables. This is achieved through the use of chance, where participants are randomly allocated to either the treatment group or the control group.

**Why the Correct Answer is Right**
The major purpose of randomization is to reduce selection bias and ensure that the groups are representative of the population being studied. This is particularly important in clinical trials where the treatment being tested may have a significant effect on the outcome. By randomizing participants, researchers can ensure that any differences between the treatment and control groups are due to the treatment itself, rather than other factors such as age, sex, or socioeconomic status. Randomization also helps to minimize confounding variables and ensures that the results of the trial are generalizable to the population being studied.

**Why Each Wrong Option is Incorrect**
**Option A:**
Randomization is not primarily used to increase the sample size of the trial. While it is true that randomization can help to increase the sample size by allowing researchers to include participants who may have been excluded from the trial due to certain characteristics, this is not the primary purpose of randomization.

**Option B:**
Randomization is not primarily used to reduce the placebo effect. While the placebo effect can be a concern in clinical trials, randomization is not the primary method used to reduce it. Other methods, such as using a blinded design, are more effective in reducing the placebo effect.

**Option C:**
Randomization is not primarily used to reduce the risk of bias in the trial. While randomization does help to reduce bias, it is not the only method used to reduce bias in clinical trials. Other methods, such as using a blinded design and ensuring that the trial is conducted in a neutral environment, are also important in reducing bias.

**Clinical Pearl / High-Yield Fact**
The CONSORT statement, which provides guidelines for the reporting of clinical trials, emphasizes the importance of randomization in clinical trials. The CONSORT statement recommends that researchers report the method of randomization used in the trial, as well as the characteristics of the participants who were randomized.

**Correct Answer: C. To reduce selection bias and ensure the groups are representative of the population being studied.**