Final answer:
The magnetic force on a positively charged particle moving in the horizontal plane towards south in a uniform magnetic field acting vertically upwards is towards the west.
Step-by-step explanation:
The direction of the magnetic force on a positively charged particle moving in the horizontal plane towards south in a uniform magnetic field acting vertically upwards can be determined using the right-hand rule. According to the right-hand rule, if the thumb of your right hand points towards the velocity of the positive charge, and the fingers point towards the magnetic field, then the direction in which the palm faces represents the direction of the magnetic force. In this case, since the positively charged particle is moving towards the south and the magnetic field is acting vertically upwards, the magnetic force will be towards the west.