The attack of elctrophilic to benzene ring giving a carbocation is the slowest and rate determining step.
Explanation: The nitration of benzene takes places in three steps, i-e. (iii) Loss of proton from carbocation giving nitrobenzene. The attack of an electrophile to benzene ring forming the carbocation is the slowest and rate determining step.
Concentrated sulphuric acid releases a molecule of SO3. The electrolphile attacks the benzene ring to form a sigma complex. The abstraction of H+ is the rate determining step.
When the electrophile adds to the aromatic ring, it produces a carbocation intermediate. The first step of electrophilic aromatic substitution is usually the rate-determining step. Since a new sigma bond forms in the first step, the intermediate is called a sigma complex.