The major product P of the given reaction is 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 SN2 mechanism, in which the nucleophile attacks the carbon atom from behind, displacing the leaving group.
In option A, the nucleophile is attacking the carbon atom from the correct side, and the leaving group is being displaced. This is the most likely pathway for the reaction to occur, and therefore option A is the correct answer.
In option B, the nucleophile is attacking the carbon atom from the wrong side. This is not a likely pathway for the reaction to occur, and therefore option B is not the correct answer.
In option C, the nucleophile is attacking the carbon atom from the correct side, but the leaving group is not being displaced. This is not a likely pathway for the reaction to occur, and therefore option C is not the correct answer.
In option D, the nucleophile is attacking the carbon atom from the wrong side, and the leaving group is not being displaced. This is not a likely pathway for the reaction to occur, and therefore option D is not the correct answer.