The molecule with largest C-X bond length is

The correct answer is Option (2) → $CH_3-I$

The C–X bond length increases as the size (atomic radius) of the halogen increases down the group.

Order of halogen size:

$\text{F} < \text{Cl} < \text{Br} < \text{I}$

Hence, C–X bond length order:

$\text{C–F} < \text{C–Cl} < \text{C–Br} < \text{C–I}$