The difference between valency and oxidation number are following except :
valency of an element is a fixed quantity whereas oxidation number varies from compound to compound
valency is a pure number whereas oxidation number has positive or negative value
oxidation number and valency are having fixed value
valency and oxidation number can be represented for an element or a radical
The correct option is D oxidation number and valency are having fixed value
Oxidation number and valency do not have fixed value as it varies from group to group and from compound to compound.