Answer:
Your brother changed the password.
Step-by-step explanation:
Number 1 is incorrect. Your brother must've asked for your permission to use your computer to complete his homework, and you accepted it.
Number 2 is incorrect. If your computer had a virus, how would you have been able to type in your login details?
Number 3 is also incorrect. The reason would be the same as the previous one.
Number 4 is relevant here. Your brother was the last one using your computer, so you would automatically assume that it was your brother that changed the password.
Hope this helped!
~Jinachi~