Final answer:
The charge on particle P when subjected to an inward force by a positively charged particle would be negative, as opposite charges attract.
Step-by-step explanation:
When a positively charged particle exerts an inward force on another particle P, we can infer that particle P must have a charge opposite to that of the positively charged particle, based on the principle that opposite charges attract each other. Therefore, the correct answer would be:
b. negative
As opposite charges attract, the charge on particle P would logically be negative in order to result in the inward force exerted by a positively charged particle.