、。
Studying is what you're supposed to do.
Literal
Study [nominalizer-のが], you [gen-の] proper-duty is.
君 is a familiar/intimate 'you,' often used by men toward equals or younger people, or in songs and aphoristic statements. Combined with 本分 ('one's true calling/proper duty'), the sentence becomes a personal exhortation — almost like a parent reminding a child. The comma sets off 勉強するのが for emphasis, giving a slight pause that makes the statement weightier.