Explanation

(c)
Coordination number is the number of ligands in the coordination sphere. Hence, the coordination number of cobalt ion in $\lbrack Co\left( H_{2}O \right)_{4}SO_{3}\rbrack Cl$ is 5.
Let the oxidation number of Co is x.
x +4(0)+($- 2$)+($-$1)=0
x +0$- 2 - 1 = 0$
x =3
Number of unpaired electrons in d-orbital are 4 because $H_{2}O$ is a weak ligand and therefore, pairing of d-electrons is not possible.