The best revision of the claim as a rebuttal to the counterclaim is:
C. Students usually study government and civics during the last two years of high school, so 16 and 17-year-olds find voting to be a way to engage with what they learn.
This option directly addresses the counterclaim by suggesting that young people, who are learning about government and civics in school, would be more motivated to engage in voting, thus potentially cultivating lifelong voting habits.