The major product of the following reaction is $\boxed{\text{A}}$.
The reaction is a nucleophilic substitution reaction, with the nucleophile being the iodide ion and the leaving group being the chloride ion. The reaction proceeds through an S_N2 mechanism, in which the nucleophile attacks the electrophilic carbon atom from behind, displacing the leaving group.
In option $\text{A}$, the nucleophile is attacking the electrophilic carbon atom from the correct side. The leaving group is also leaving from the correct side, so this is the major product.
In option $\text{B}$, the nucleophile is attacking the electrophilic carbon atom from the wrong side. This will result in a product that is not as stable as the product in option $\text{A}$, so it will be formed in a lower yield.
In option $\text{C}$, the nucleophile is attacking the electrophilic carbon atom from the correct side, but the leaving group is leaving from the wrong side. This will result in a product that is not as stable as the product in option $\text{A}$, so it will be formed in a lower yield.
In option $\text{D}$, the nucleophile is attacking the electrophilic carbon atom from the wrong side, and the leaving group is leaving from the wrong side. This will result in a product that is not as stable as the product in option $\text{A}$, so it will be formed in a very low yield.