a
In \(\left[ Co \left( NH _{3}\right)_{5} CO _{3}\right] ClO _{4}\), five ammonia ligands and one carbonate ligand is attached to the central cobalt atom. Hence, the coordination number is six.

Let \(X\) be the oxidation number of cobalt. \(X+(-2)+(-1)=0\) or \(X=3 .\) Hence, the oxidation number is 3 .

The outer electronic configuration of cobalt (atomic number 27 ) is \(3 d^{7} 4 s^{2} .\) The outer electronic configuration of \(Co ^{+3}\) will be \(3 d^{6} 4 s^{0} .\) Thus it contains six d electrons.

\(\left[ Co \left( NH _{3}\right)_{5} CO _{3}\right] ClO _{4}\) is inner orbital or low spin complexes. all the electrons are paired due to \(d^{2} s p^{3}\) hybridization of \(C o^{+3}\) ion. Hence, the number of unpaired d electrons is zero.