Final answer:
The dependent variable in this experiment would be the pain levels experienced by the subjects.
Step-by-step explanation:
In this study, the dependent variable would be d) The pain levels experienced by the subjects. The dependent variable is the variable that is being measured or observed in an experiment, and in this case, it is the pain levels experienced by the subjects. The researchers are investigating the effectiveness of different pain relievers, so they would be measuring the pain levels as the dependent variable to determine which pain reliever is most effective.