Answer
See explanation below.
Work Step by Step
a.) P forms P^3- with a -3 charge. [P^3-] = 1s2 2s2 2p6 3s2 3p6
b.) Mg forms Mg^2+ with a +2 charge. [Mg2+] = 1s2 2s2 2p6
c.) Se forms Se^2- with a -2 charge. [Se^2-] = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6