The ion 12B3+ consists of?

It's d. The number in front (12) is the mass number, which is the sum of protons and neutrons. The atomic number, or number of protons, for B is 5. 12-5 is the number of neutrons (7). Since it has a charge of 3+, it has three more protons than electrons (because of their charges). 5-3 is the number of electrons (2).