The bond dissociation energy is highest for

🧠 Bond dissociation energies in halogens: $\ce{F2}$ is anomalously weak despite F being the most electronegative — the very small F–F bond has large lone pair–lone pair repulsions between the compact fluorine atoms.

| Halogen | Bond energy (kJ/mol) | |---|---| | $\ce{F2}$ | ~159 (weakest — lone pair repulsion in small atoms) | | $\ce{I2}$ | ~151 (weak — large, diffuse orbitals, poor overlap) | | $\ce{Br2}$ | ~193 | | $\ce{Cl2}$ | ~243 (highest — optimal size for overlap without excessive repulsion) |

$\ce{Cl2}$ has the largest bond dissociation energy because Cl atoms are large enough to reduce lone pair repulsion (unlike F) but small enough for effective orbital overlap (unlike I and Br).

$\boxed{\text{Answer: (a) } \ce{Cl2}}$