Both assertion and reason are correct and reason is the correct explanation of assertion
Explanation Electrolysis of NaCl is represented by following chemcial reactions
At cathode
`H^(+)(aq)+e^(-) rarr(1)/(2)H_(2)(g)`
At anode `Cl^(-)(Aq)rarr(1)/(2)Cl_(2)+e^(-),E_("cell")^(@)=1.36V`
`2H_(2)O(aq)rarrO_(2)(g)+4H^(+)(aq)+4e^(-):E_("cell")^(@)=1.23V`
`E_("cell")^(@)` for this reaction has lower value but formation of oxygen at anode requires over potential.