One option is trade offs. Tell her if she study’s for an hour, you’ll do something she likes. That sometimes works for me.

The other option is to just be as gentle as possible, bring it up in a joke(risky). If she’s just trying to get motivated, tell her about all the cool things she’ll be able to do or become once she finishes classes, while telling her to study.

All this being said if she is just content with getting a C or B in the class and is confident in her current knowledge to get that, then maybe just let it be.