Am I allowed to choose any class I want in a College/University? Let's say I wanted only science-based courses even though I had an A in Chemistry and C in Biology. I need a majority of my classes to be science-based.

You have certain lower division requisites to fulfill. Social science, humanities and the like. Most can be gotten over quickly and some have HS carry over.

Most of you classes will be science ( don't forget the math! ) but I am troubled by that C in my area of interest. There is one science and that is biology and there is one biology and that is evolutionary biology!