Answer:
The answer is: a. straight up.
The magnetic force on a moving charged particle is given by the following equation:
F=qvBsinθ
where:
F is the magnetic force
q is the charge of the particle
v is the velocity of the particle
B is the strength of the magnetic field
θ is the angle between the velocity of the particle and the magnetic field
In this case, we know that the particle is moving to the west and experiences zero force. This means that the angle between the velocity of the particle and the magnetic field must be 90 degrees. In other words, the magnetic field must be pointing straight up.
Here is a diagram to illustrate:
[Diagram of a charged particle moving to the west and experiencing zero force]
The magnetic field is represented by the blue arrows. The velocity of the particle is represented by the red arrow. The force on the particle is represented by the green arrow. As you can see, the force on the particle is zero because the magnetic field is pointing straight up.