Final answer:
A positively charged object becomes neutral when grounded by gaining electrons from the earth, as it provides a vast source of electrons.
Step-by-step explanation:
The best explanation for how a positively charged object can become neutral while being grounded is that the object gains electrons from another object (A). When an object is grounded, it allows for the transfer of electrons to equalize its charge with the ground. If the object is positively charged, grounding provides a path for electrons to flow from the earth (which is a huge reservoir of charge) into the object, thus neutralizing its positive charge. This process is an example of ionization, where atoms or objects gain or lose electrons and as a result, become charged or neutral ions.