Final answer:
Using the right-hand rule, the direction of the magnetic force on a positively charged particle moving south in an upward magnetic field is west.
Step-by-step explanation:
The direction of the magnetic force on a positively charged particle moving in a uniform magnetic field can be determined using the right-hand rule. Given that the positively charged particle is moving towards the south and the uniform magnetic field is acting vertically upwards, we need to point our right hand's fingers in the direction of the particle's velocity (south), curve them towards the direction of the magnetic field (upwards), and the thumb will then point in the direction of the force. In this case, the thumb would point to the west, indicating that the magnetic force on the positively charged particle is directed towards the west. Therefore, the correct answer is (b) West.