In the case of Zn, it is likely eligible for one charge, as its 3d is full, it can lose the electrons from the 4s2 level that it does to become Zn2+.
Iron has 6 electrons in the 3d energy level, which signifies that it would give up 6+2 to get to the next stable energy level. It is much convenient for it to give up the 4s2 and then one more out of the 3d6 to produce Fe2+ and Fe3+ respectively.
Sodium has one electron to give away in the 3s energy level, as soon as it gives its one electron, it will look like Ne. In silver, the 4d10 energy level is full, it can only give up the 5s1 electron easily.
In the case of cobalt, the electrons can be taken from the 4s2 energy level and then one more out of the 3d7 energy level, and after that more and more again. Thus, it can go from +5 to -1 charge, due to much more happening in the 3d7 energy level.
Therefore, cobalt can likely form ions with multiple charges.
