The arenium ion which is not involved in the bromination of Aniline is

During electrophilic bromination, aniline first donates its lone-pair electrons to the aromatic ring through resonance. This makes the ring highly activated toward electrophiles and directs the attack exclusively to the ortho and para positions.

The key mechanistic intermediate of any aromatic electrophilic substitution is the σ-complex (also called the arenium ion). For aniline these σ-complexes must satisfy two conditions:
1. The electrophile attaches at an ortho or para carbon (because $$-NH_2$$ is an ortho/para director).
2. At least one resonance form of the σ-complex must place the positive charge on the nitrogen atom. Such resonance delocalisation greatly stabilises the intermediate.

If the electrophile were to attack at the meta position, none of the resonance structures would be able to shift the positive charge onto the nitrogen atom; the charge would remain on ring carbons only. Consequently, the meta σ-complex is far less stabilised and is not formed under ordinary conditions.

Among the four structures given in the options, the three that correspond to ortho or para attack all allow resonance forms in which the positive charge resides on nitrogen, so they are genuine intermediates. The remaining structure (Option C) shows the electrophile attached at the meta position; its resonance forms keep the positive charge on ring carbons and never on nitrogen. Therefore it cannot arise during bromination of aniline.

Hence, the arenium ion that is not involved in the bromination mechanism of aniline is the meta σ-complex shown in Option C.

Option C which is: (meta σ-complex)