Oxidation number:
It is the number that represents an elements ability to lose or gain electrons.
The answer is NaClO₃, that is in this compound , the oxidation state of Cl is +5.
That can be calculated as follows:
Suppose x denotes Cl
So its oxidation number will be:
1+x+3\times -2=0
so x calculated comes out to be 5. So the answer is +5 that is the oxidation number of Cl in NaClO₃, will be +5.