In this case, since rubidium is in group IA, one electron will be eligible to pair during a chemical reaction. In addition, as long as bromine is in group VIIA, when pairing with a metal, just one electron is eligible to pair as well. Now, based on the aforesaid facts, we infer that the undergoing chemical reaction will be:
So the formed product is which is rubidium bromide, that is the c) answer.