Which total mass is the largest? (A) The mass of 2 electrons
(B) The mass of 2 neutrons
(C) The mass of 1 electron plus the mass of 1 proton
(D) The mass of 1 neutron plus the mass of 1 electron

Nonmetals usually react by gaining electrons

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.