Which of the following has the highest bond dissociation energy?
 

There is a significant difference in electronegativity between the C and X atoms because of the high electronegativity of the halide ion (denoted by X-). As a result, the C-X bond has a substantial polar character and is not entirely covalent. As we proceed from fluorine to iodine in the group, the C-X bond length rises as the size of the halide ion grows down the group.