Cl2(s); oxidation number 1 is the incorrect choices in oxidation number.
Explanation:
In the elemental form oxidation state is zero. Here chlorine is present in elemental form so oxidation state is zero.
Oxidation number depends on the number of electrons gained or lost by an atom of the element say in compound formation.
If electron is gained oxidation number becomes negative.
If electron is lost then oxidation number is positive.
If the octet rule is fulfilled that valence shell is filled them atomic number gets zero. Since Cl2 is in neutral state the oxidation number is 0.
Oxidation number in general can be made out by checking the valency of the element as oxidation number is also equal to the valency.
