Q.The compound having highest carbon-halogen (C - X) bond length is:

Carbon–Halogen Bond Length and Strength Trend

In the haloalkanes the carbon–halogen ($\text{C–X}$) bond changes systematically as we go down group 17 from fluorine to iodine. As the halogen atom gets larger, its bonding electrons occupy larger, more diffuse orbitals held farther from the nucleus, so the atomic (covalent) radius increases: $\text{F} < \text{Cl} < \text{Br} < \text{I}$.

Two consequences follow:

Bond length increases. Because the halogen is bigger, the shared electron pair is spread over a longer distance, so the $\text{C–X}$ bond length grows down the group. Representative values: