which is more reactive towards nitration  and why ----flurobenzene ,chlorobenzene,bromobenzene or iodobenzene

For the given haloarenes, the order of nitration is as follows

 Iodobenzene > Bromobenzene > Chlorobenzene > Fluorobenzene

This is because the halogen group, though ortho - para directing, is also deactivating because of the -I effect. More is the electronegativity of the halogen atom, more is the -I effect and more is the deactivation of the benzene ring towards electrophilic substitution. Since fluorine is the most electronegative, it deactivates the benzene ring the most and hence nitration occurs the slowest in fluorobenzene. Iodine, on the other hand being less electronegative deactivates the ring to a lesser extent and therefore nitration proceeds the fastest in iodobenzene.