The decreasing order of reactivity of alkyl halides is based on the ease with which they undergo nucleophilic substitution reactions.

Order of Reactivity:

The alkyl halides can be arranged in the decreasing order of reactivity as follows:

R-I > R-Br > R-Cl

Explanation:

The reactivity of alkyl halides towards nucleophilic substitution reactions is influenced by the following factors:

1. The nature of the halogen:

The reactivity of alkyl halides decreases in the order: iodide > bromide > chloride. This is because the size of the halogen increases down the group, leading to a decrease in the bond strength and an increase in the polarizability of the C-X bond. As a result, the carbon-halogen bond becomes more susceptible to nucleophilic attack.

2. The nature of the alkyl group:

The reactivity of alkyl halides increases with increasing branching of the alkyl group. This is because the steric hindrance caused by the bulky alkyl groups makes it difficult for the nucleophile to approach the carbon atom, leading to a decrease in the reaction rate.

3. The nature of the nucleophile:

The reactivity of alkyl halides towards nucleophilic substitution reactions is also influenced by the nature of the nucleophile. Generally, stronger nucleophiles are more reactive towards alkyl halides.

Based on the above factors, the decreasing order of reactivity of alkyl halides is:

R-I > R-Br > R-Cl

Hence, option B is the correct answer.