(N/A) The $\sigma$-complex (also known as the arenium ion) is formed when an electrophile $(E^+)$ attacks the benzene ring.
$1$. Resonance structures: The positive charge is delocalized over the ortho and para positions relative to the site of electrophilic attack. The three resonance structures are:
$(i)$ The positive charge is at the ortho position.
(ii) The positive charge is at the para position.
(iii) The positive charge is at the other ortho position.
$2$. Stability: The $\sigma$-complex is stabilized by resonance,which delocalizes the positive charge over the ring carbons. However,the aromaticity of the benzene ring is lost during the formation of the $\sigma$-complex,making it a high-energy intermediate. The resonance energy of the benzene ring is significantly higher than the stabilization energy gained by the delocalization of the positive charge in the $\sigma$-complex.