Explain  why  the  order of reactivity of  haloalkanes is RI>RBr>RCl.........

On going down the group size of the halides increases and hence stability of the compound decreases as bond length is going to increase resulting in easy break down of the bond and hence reactivity increases. 

Thus the correct order of the reactivity is:

R-I >R-Br> R-Cl> R-F