Answer:
C. a controlled experiment
Explanation:
The controlled experiment requires that participating subjects are divided into two groups. One group would be exposed to the influence of the factor (independent variable) that the researcher wants to investigate (here, the extra handling and enriched environment) and the other group would be not. As the result, the effects of the factor under investigation on the dependent variable (here, how quickly rats learned a maze) can be compared to the control group and thus measured.