The answer is :
2H₂O (l) ------>2H₂ (g) + O₂ (g)
That is ,
Oxidation number of oxygen increases in reaction :
2H₂O (l) ------>2H₂ (g) + O₂ (g)
In H₂O, oxidation number of oxygen is -2 but in O₂ molecule it is 0.
So, oxidation number of oxygen increases in this reaction that is from -2 to 0.
So in the reaction that is 2H₂O (l) ------>2H₂ (g) + O₂ (g) , oxidation number of oxygen increases .