Is it a good idea to retake classes you got a C in?

I screwed up Sophomore year with Algebra 2 by forgetting to turn in a huge project and got a C 2nd semester since late work was not accepted.

Should I retake Algebra 2 or just take another class?
I was planning on taking Music History if I don’t retake Algebra 2. I have an A in precalc right now so it’s not like I didn’t slack in Algebra 2, just that the project destroyed my grade.

Lets assume I will get an A for either class. Which one will be better?

many colleges will not recalculate your GPA if you repeat the course that you passed. For example the UC system in CA has that policy.

take music history

By taking math courses more advanced than algebra 2 and by getting good grades in these courses you’ve convincingly proved that you’ve mastered the material in algebra 2. Retaking the course (would your school even allow that?) you’re proving little. Surely you’ll get an A. But academically it’s pointless. Move on.

No, waste of time if you are doing well in more advanced courses of same subject. In fact, if you get ahead one year in math over the summer for example, this will look even more trivial.

The only reason to retake would be if you felt that you had failed to grasp the material sufficiently. That wasn’t the case here. Don’t repeat. Just do better next year. Turning your math grade around in your junior year will actually look impressive (not as impressive as two As, but lets make lemonade here…)