Answer
$a) Rb$
$b) N^{3-}$
$c) Cr^{3+}$
Work Step by Step
$a) Rb^{+} $is smaller because since it has lost an electron, there are more protons then electrons and thus the presence of more protons will pull the electrons closer, decreasing the atomic radius
$b) N^{3-}$ is larger because it has more electrons than protons. This results in more shielding and more repulsion which increases the atomic radius.
$c) Cr^{3+}$ is larger because it has lost fewer electrons than $ Cr^{6+}$ Thus the shielding in $Cr^{3+}$ is larger than $ Cr^{6+}$ and thus it has more repulsion and it is bigger
