Answer:

To determine the oxidation state of the metal in each complex, we need to consider the charges carried by the ligands and the overall charge of the complex.

In complex a) [Pt(NH3)Cl], we have one ammine ligand (NH3) and one chloride ligand (Cl). The overall charge of the complex is not given, but we can assume it is neutral since no charge is specified. The oxidation state of the ligands can be determined by considering the electronegativity of the atoms involved. Nitrogen has an electronegativity of 3.04, while hydrogen and chlorine have electronegativities of 2.20 and 3.16, respectively. Assuming that the ammine ligand donates its lone pair of electrons to the platinum atom, nitrogen would have a -3 charge and platinum would have a +3 charge. Chlorine would have a -1 charge. Therefore, the overall charge of the complex would be -1. Since platinum has a +3 charge, the oxidation state of the metal in complex a) is +3.

In complex c) [Cr(NH3)5Cl], we have five ammine ligands (NH3) and one chloride ligand (Cl). Assuming that each ammine ligand donates its lone pair of electrons to the chromium atom, nitrogen would have a -3 charge and chromium would have a +3 charge. Chlorine would have a -1 charge. Therefore, the overall charge of the complex would be -1. Since chromium has a +3 charge, the oxidation state of the metal in complex c) is +3.

In complex d) [Cr(en)3]Cl3, we have three ethylenediamine ligands (en). Assuming that each ethylenediamine ligand donates its lone pair of electrons to the chromium atom, nitrogen would have a -3 charge and chromium would have a +3 charge. Therefore, the overall charge of the complex would be neutral. Since chromium has a +3 charge, the oxidation state of the metal in complex d) is +3.

In complex b) [Cr(CO)6], we have six carbonyl ligands (CO). The overall charge of the complex is not given, but we can assume it is neutral since no charge is specified. The oxidation state of the ligands can be determined by considering the charge distribution in carbon monoxide. Carbon has an electronegativity of 2.55, while oxygen has an electronegativity of 3.44. Carbon monoxide contains a triple bond between carbon and oxygen, with a partial positive charge on carbon and a partial negative charge on oxygen. Assuming that each carbonyl ligand donates its lone pair of electrons to the chromium atom, carbon would have a -2 charge and chromium would have a +2 charge. Therefore, the overall charge of the complex would be neutral. Since chromium has a +2 charge, the oxidation state of the metal in complex b) is 0.

Therefore, the correct answer is option b) [Cr(CO)6], where the oxidation state of the metal is 0.