Biology

The correct answer is:

To ensure the dependent variable is not affected by multiple factors.

Having only one independent variable in an experiment allows you to clearly establish a cause-and-effect relationship. If you change more than one variable at the same time, it becomes difficult to determine which factor caused any observed changes in the dependent variable.

By keeping the independent variable isolated, you can confidently conclude that any changes in the dependent variable are due to the manipulation of that specific independent variable.

To clarify the other options: