Answer:
C
Explanation:
The correct answer would be the group that was given a placebo.
A control group in an experimental study is the group that is subjected to all other conditions except the variable whose effects are being determined during the course of the experiment.
Since the main goal of the research is to evaluate the effectiveness of the HPV vaccine, the difference between the experimental and the control group would be the vaccine. Instead, the control group would be given a placebo - a substance that is designed not to have any effect but looks like the vaccine itself.
Correct option: C