Answer:
D. Logos
Step-by-step explanation:
Bias is defined as the "prejudice in favor of or against one thing, person, or group compared with another, usually in a way considered to be unfair."
Ethos is an ethical appeal and Pathos is an emotional one.
It clearly isn't any of those mentioned above.
Studying makes you know more about a subject, which improves your chances of answering questions correctly, which by consequence, creates a scenario in which it is more likely for you to get a good grade. This is a logical appeal, therefore, it is Logos.