**Reactivity of Halogen Atom**

The reactivity of a halogen atom depends on its ability to undergo substitution reactions, which is determined by the strength of the carbon-halogen bond. The strength of the carbon-halogen bond is influenced by various factors, including the electronegativity of the halogen and the nature of the carbon-halogen bond (i.e., primary, secondary, or tertiary).

**Electronegativity of Halogens**

The reactivity of halogens generally decreases as we move down the periodic table. This is because electronegativity decreases down the group, leading to a weaker attraction between the halogen atom and the carbon atom in the carbon-halogen bond. As a result, the carbon-halogen bond becomes weaker, making it more susceptible to substitution reactions.

**Effect of Carbon-Halogen Bond Strength**

The strength of the carbon-halogen bond is influenced by the nature of the carbon-halogen bond. In general, primary alkyl halides (where the halogen is bonded to a primary carbon atom) have weaker carbon-halogen bonds compared to secondary and tertiary alkyl halides. This is because the electron density around the primary carbon atom is higher, making it more susceptible to attack by nucleophiles.

**Analysis of Options**

a) Propyl chloride: In propyl chloride, the halogen atom (chlorine) is bonded to a primary carbon atom. Therefore, the carbon-halogen bond is relatively weak, making it more reactive compared to the other options.

b) Propyl iodide: In propyl iodide, the halogen atom (iodine) is bonded to a primary carbon atom. Since iodine is a larger atom compared to chlorine, the carbon-iodine bond is weaker. Therefore, propyl iodide is more reactive than propyl chloride.

c) Isopropyl chloride: In isopropyl chloride, the halogen atom (chlorine) is bonded to a secondary carbon atom. The carbon-halogen bond is stronger compared to propyl chloride, making it less reactive.

d) Isopropyl bromide: In isopropyl bromide, the halogen atom (bromine) is bonded to a secondary carbon atom. Bromine is a larger atom compared to chlorine, so the carbon-bromine bond is weaker than the carbon-chlorine bond. Therefore, isopropyl bromide is more reactive than isopropyl chloride.

**Conclusion**

Based on the analysis above, the reactivity of the halogen atom is minimum in isopropyl chloride (option c). This is because the carbon-halogen bond is stronger in isopropyl chloride compared to the other options, making it less susceptible to substitution reactions.

A) propyl chloride.

becoz when carbonium is formed by isopropyl group it is more stable than that formed by n propyl group thus there reactivity will be more. This eliminates option C and D.

Now between A and B, -I is more reactive than -Cl.

this gives us propyl chloride as least reactive.