o-Phenylenediamine $\stackrel{{\mathrm{HNO}}_2}{\to }\underset{\mathrm{Major}\mathrm{product}}{\text{'X'}}$. 'X' is 

The reaction involving o-Phenylenediamine and HNO₂ (nitrous acid) typically leads to the formation of a diazonium salt. In the case of o-Phenylenediamine, the reaction is a diazotization reaction, which forms a diazonium ion at the amino group (-NH₂) on the benzene ring. The major product 'X' after the reaction with HNO₂ would be:

• X is a diazonium salt, specifically 2-diazo-1,3-benzenediamine (where the diazonium group is attached at the position ortho to the amino group).