Which best describes the purpose of a controlled experiment?
a.to make sure the results of the experiment support the hypothesis
b.to prevent a dangerous condition resulting from a reaction
c.to examine whether one variable causes a change in another
d.to prevent an undesired change in a variable

I think the correct answer from the choices listed above is option C. The purpose of a controlled experiment is to  examine whether one variable causes a change in another. An independent variable is the only factor that is allowed to be adjusted, with the dependent variable as the factor that the independent variable will affect.

C. to examine whether one variable causes a change in another.

Explanation:

A controlled experiment is the one where a control variable is used which is used as a benchmark or standard for comparing the changes occurring in a dependent variable due to the changes occurring in an independent variable by undergoing manipulation and alteration.

On the basis of the above explanation, C. to examine whether one variable causes a change in another is the correct option.

Altering the amount of nitrogen that is stored in the biosphere.