The products of ozonolysis of 1,2-dimethyl benzene (o-xylene) are -

1. Methylglyoxal, 1, 2-dimethylglyoxal, and glyoxal

2. Methylglyoxal, 1, 3-dimethylglyoxal, and glyoxal

3. Methylglyoxal, 1, 4-dimethylglyoxal, and glyoxal

4. Methylglyoxal, 1, 5-dimethylglyoxal, and glyoxal

Hint: Aromatic electrophilic substitution based nitration and concept of ozonolysis
 
Explanation:
 
Step 1:
 
o-xylene has two resonance structures, both give the different products in the ozonolysis reaction. The reaction is as follows:
 
 
 
  Step 2:
 
All three products, i.e., methylglyoxal, 1,2-dimethylglyoxal, and glyoxal are obtained from two Kekule structures. Since all three products cannot be obtained from any one of the two structures, this proves that o-xylene is a resonance hybrid of two Kekule structures (I and II).