Answer
Coordination number: $4$
Oxidation number: $+2$
Work Step by Step
The ligands ($Cl_{4}$) that surround the central metal atom $Cd$ are 4. Thus, the coordination number is 4.
Let us suppose the oxidation number of Cd is $X$. To find the oxidation number, we sum the known oxidation numbers and equal it to the charge on the complex.
$Na _{2} = 2 \times (+1) $
$Cl _{4} = 4\times (-1) $
The overall charge is zero on the complex, thus:
$ 2\times (+1) +X + 4 (-1) = 0 $
$ +2 +X -4 = 0$
$X= +4-2$
$X = +2 $
