I got an 16/26, D-, on my final exam. I had a B, 86%. Will I end up with a C or lower for my final grade?
I'm sorry to tell you this, but there is no way to know. There is other information that I need to know in order to calculate the final grade. I would need to know how your school or teacher calculates your final grade. Is the exam worth a portion of your 4th quarter grade? Is it separate from your quarter grades and then calculated into your final semester grade?
Likely the later is true. If that's the case, how much are exams worth? At my school, the exam is worth 20% of the final grade. Each quarter is worth 40%. IF that is how your school does it, I still need to know what your 3rd quarter grade is.
I'm going to assume that your current 86% is your 4th quarter grade. I will also assume (for the sake of calculation) that your 3rd quarter grade is also an 86%. You scored a 62% on your exam. With those scores in the 40-40-20 grading system, your final grade will be an 81%. That's likely a B-. The reason I say likely is because your school's grading scale might be different.
Long story short, I don't think a "C" or lower is very possible.