Presence of a nitro group in a benzene ring:

The nitro group (${\text{-NO}}_2$) is a strong electron-withdrawing group due to both $-I$ effect and $-M$ (mesomeric) effect.It withdraws electron density from the benzene ring, especially from $ortho$ and $para$ positions. This makes the ring electron-deficient.Since electrophiles are electron-seeking species, they are less likely to attack an electron-deficient ring. Therefore, the ${\text{-NO}}_2$ group deactivates the ring towards electrophilic aromatic substitution.