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

The correct option is NO₂⁺.

Detailed Explanation:
The nitration of benzene is a classic example of an electrophilic aromatic substitution reaction. In this process, benzene is treated with a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄), commonly referred to as the nitrating mixture. The reaction proceeds through the generation of a strong electrophilic species that can attack the stable, electron-rich aromatic system of the benzene ring.

The role of concentrated sulfuric acid in this mixture is to act as an acid catalyst to protonate the nitric acid and generate the active electrophile. The step-by-step mechanism of the formation of the nitrating species is as follows:

Step 1: Protonation of Nitric Acid
Since sulfuric acid is a much stronger acid than nitric acid, it donates a proton (H⁺) to nitric acid. In this step, nitric acid acts as a base. The protonation occurs at the hydroxyl oxygen of the nitric acid molecule, forming a protonated nitric acid intermediate:

${\mathrm{HNO}}_3+H_2{\mathrm{SO}}_4\rightleftharpoons H_2{\mathrm{NO}}_3^{+}+{\mathrm{HSO}}_4^{-}$

Step 2: Dissociation to Form the Electrophile
The protonated nitric acid intermediate is unstable because it contains an excellent leaving group (a water molecule). It undergoes heterolytic cleavage to release a neutral water molecule and generate the highly reactive nitronium ion (${\mathrm{NO}}_2^{+}$):

$H_2{\mathrm{NO}}_3^{+}\to {\mathrm{NO}}_2^{+}+H_{2}O$

The released water molecule is immediately protonated by another molecule of sulfuric acid to form a hydronium ion (H₃O⁺) and a bisulfate ion (HSO₄^-).

Overall Reaction for Electrophile Generation:

${\mathrm{HNO}}_3+2H_2{\mathrm{SO}}_4\rightleftharpoons {\mathrm{NO}}_2^{+}+H_3{O}^{+}+2{\mathrm{HSO}}_4^{-}$

The resulting nitronium ion (${\mathrm{NO}}_2^{+}$) is the active nitrating species (electrophile) that undergoes electrophilic attack on the benzene ring to yield nitrobenzene.