Q.Which one of the following reagents is used in Gattermann reaction?

In a haloarene, the halogen sits on an $sp^2$ ring carbon whose lone pair can conjugate by resonance with the ring's $\pi$ system, delocalising electron density from the halogen into the ring and giving the $\text{C--X}$ bond partial double-bond character. This shortens the bond (e.g. $\text{C--Cl}$ falls from about $1.77$--$1.78\ \text{\AA}$ in a saturated haloalkane to about $1.69\ \text{\AA}$ in chlorobenzene) and strengthens it, reinforced independently by the greater $s$-character of the $sp^2$ ring carbon's bonding orbital. Both $S_N2$ (blocked by the ring itself, with no accessible backside, and by the electron density resonance donation adds to the ring carbon) and $S_N1$ (blocked because an aryl cation' …