Don’t try to to “explain” the grade, test score, etc. - that would just make it stand out. It is what it is.
It’s obviously not as good as an A or B, but it’s one grade of probably 30, on one part of your application. Every applicant will have strong and weak parts on the application, this is one very small weak note on yours.
You should make your essay as strong as you can, regardless of your grade in Biology.
Also, fwiw, a grade on a 3 hour test won’t “offset” a yearlong course grade. Many admissions offices don’t even look at AP scores. They are intended for “Placement” - hence the name.