Answer
See the answer below.
Work Step by Step
a) Fe, group 8, period 4
$[Ar]\ 3d^6\ 4s^2$
$Fe^{2+}: [Ar]\ 3d^6$
$Fe^{3+}: [Ar]\ 3d^5$
b) $Co^{2+}, Al^{3+}, Sn^{4+}$
Co, group 9, period 4
$[Ar]\ 3d^7\ 4s^2$
$Co^{2+}: [Ar]\ 3d^7$, paramagnetic, three unpaired electrons.
Al, group 13, period 3
$[Ne]\ 3s^2\ 3p^1$
$Al^{3+}: [Ne]$, diamagnetic
Sn, group 14, period 5
$[Kr]\ 4d^{10}\ 5s^2\ 5p^2$
$Sn^{4+}: [Kr]\ 4d^{10}$, diamagnetic