In nitration of benzene using a mixture of conc. H2SO4 and conc. HNO3, the species that initiates the reaction is :

1. NO2
2. NO+
3. NO2+
4. NO2-

Hint: The electrophile is nitronium ion 
Nitration of benzene using a mixture of conc. H2SO4 and conc. HNO3 proceeds as
This reaction is known as electrophilic aromatic substitution reaction.