The correct option is B R−I>R−Br>R−Cl>R−F
F,Cl,Br and I belong to the same group. Atomic radii go on increasing down a group. As the halogen becomes larger on going down the group, R−X bond also becomes weaker. The decreasing order of bond length. is C−I>C−Br>C−Cl>C−F.
During dehydrohalogenation C−I bond breaks more easily than C−F bond. So reactivity order of halides is
R−I>R−Br>R−Cl>R−F.