The correct option is C C6H5−I
Due to -I effect of X-group, it reduces electron density of benzene, so halobenzene is less reactive than benzene for electrophilic substitution reaction.
As electronegativity of F is the highest among halogens, so its -I effect is highest.
So, reactivity of halobenzene compounds towards electrophillic substitution is Ph - I > Ph - F.
-I effect order is as follows:
−NO2>−CN>−F>−I
Hence, C6H5−CN and C6H5−NO2 will be less reactive towards electrophilic substitution than halobenzene.
Hence, the correct answer is option (c).