Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A): R–I undergoes S_N2 reaction faster than R–Cl.

Reason (R): Iodine is a better leaving group because of its large size.

In the light of the above statements, choose the correct answer from the options given below:

Let us evaluate the given Assertion (A) and Reason (R) step-by-step:

1. Analysis of Assertion (A):
The assertion states that alkyl iodides ($R-I$) undergo nucleophilic substitution bimolecular ($S_{N}2$) reactions faster than alkyl chlorides ($R-Cl$).
In an $S_{N}2$ reaction, the rate-determining step involves the simultaneous attack of the nucleophile and the departure of the leaving group (halide ion). The rate of the reaction depends heavily on the leaving group ability of the halide. Since the iodide ion ($I^{-}$) is a much better leaving group than the chloride ion ($C^{-}$), alkyl iodides react significantly faster than alkyl chlorides. Therefore, Assertion (A) is true.

2. Analysis of Reason (R):
The reason states that iodine (iodide ion) is a better leaving group because of its large size.
As we move down Group 17 in the periodic table ($F\to Cl\to Br\to I$), the atomic and ionic size increases. The iodide ion ($I^{-}$) has a larger size compared to the chloride ion ($C^{-}$). Because of its large size, the negative charge on the iodide ion is dispersed over a larger volume, making it highly stable and a very weak base. Since weaker bases are better leaving groups, $I^{-}$ is an excellent leaving group. Thus, Reason (R) is true.

3. Linking Assertion and Reason:
Since the superior leaving group ability of iodine (due to its large size and stable charge dispersion) is the precise reason why alkyl iodides undergo $S_{N}2$ reactions faster than alkyl chlorides, Reason (R) is the correct explanation of Assertion (A).

Therefore, both A and R are true and R is the correct explanation of A.