B. correctly when on a north or south heading.
In the Northern Hemisphere while accelerating or decelerating on a north or south heading, the magnetic compass will normally indicate correctly. Because of the way it is mounted, the aft part of the compass card tilts upward while accelerating and tilts down when decelerating on an east or west heading. To help you remember this think of ANDS - Accelerate North, Decelerate South.
Answer A is incorrect because compass acceleration and deceleration errors do not occur on a north or south heading, so a momentary turn would not be normal in that case. Answer C is incorrect because a turn to the south only results from deceleration, not acceleration.