Final answer:
A positively charged ball only experiences a magnetic force if it is moving relative to a magnetic field. Therefore, only observers in motion relative to the ball would observe such a magnetic force acting on the ball.
Step-by-step explanation:
In physics, particularly electromagnetism, a positively charged object only experiences a magnetic force if it is moving relative to a magnetic field. This is due to the Lorentz force, which states that the force felt by a charge moving in a magnetic field is proportional to the charge, the velocity of the charge, and the strength of the magnetic field. Therefore, a positively charged ball held by observer B will not experience a magnetic force unless both the ball and the observer are moving relative to the magnetic field.
For an observer at rest relative to the ball, no magnetic force acts on the ball. However, for observers in motion relative to the ball, they would see the ball moving and, depending on the presence and orientation of a magnetic field, a magnetic force could act on the ball. This is in accordance with the principles of physics that relate charge motion to magnetic forces. Hence, the correct answer to the question would be c) Observers in motion relative to the ball.