Angle B is opposite _____ help please!!

Hence Angle B is opposite to line segment AC.
Angle:
It is the space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
So the side which is not included in the formation of the angle B is the opposite side to the angle or we may say that the angle is opposite to a side.
Hence Angle B is opposite to line segment AC.