Question

an experiment that tests only one variable at a time by using a comparison of a control group and an experimental group is?

an independent variable

a controlled experiment

a theory

a dependent variable

Explanation:

The question describes an experiment with one tested variable, control group, and experimental group. An independent variable is the tested factor, not the experiment type. A theory is a well-supported explanation, not an experiment. A dependent variable is measured, not the experiment. A controlled experiment matches the description: testing one variable via control and experimental groups.

Answer:

a controlled experiment