The main factor which decides the reactivity of (R-X) is

Explanation

The main factor that decides the reactivity of alkyl halides is the bond strength of the C-X bond. The strength of the carbon-halogen bond (C-X) is a crucial factor. Weaker bonds are generally more reactive because they are easier to break. As you move down the halogens in a given alkyl group, the bond strength typically decreases. For example, in a series like CH3-X, the reactivity increases as you go from F to Cl to Br to I. So, the weaker the C-X bond, the more reactive the alkyl halide is likely to be in substitution or elimination reactions. This reactivity trend is commonly observed in nucleophilic substitution and elimination reactions involving alkyl halides.