The correct answer is C. A controlled experiment
Explanation:
A controlled experiment occurs as a researcher or scientist controls one or more variables to test a hypothesis. Because of this, in a controlled experiment it is common to find a control group with subjects that do not receive a treatment or that are under regular environment conditions and an experimental group with subjects that have a different treatment and whose conditions are modified on purpose for the researcher to compare the results with the control group.
In the case presented, a controlled experiment takes place because the psychologist divided the group of rats into a control group and an experimental group (rats with extra handling and enriched environment). This also shows the psychologist controls one variable to test a hypothesis which in this case is related to learning.
