Which of the following alkyl halides is used as a methylating agent?

\(CH_3I\)

The correct answer is option 3.\(CH_3I\).

Methylating agents are compounds that can donate a methyl group \((CH_3)\) to another molecule. The most common methylating agent is methyl iodide \((CH_3I)\). Methyl iodide is used in a variety of reactions, including the synthesis of organic compounds and the modification of DNA.

The other alkyl halides in the answer choices are not as effective methylating agents as methyl iodide. Chlorobenzene \((C_6H_5Cl)\) and ethyl chloride \((C_2H_5Cl)\) are less reactive than methyl iodide because the chlorine and ethyl groups are more electron-withdrawing than the iodine group. Bromoethane \((C_2H_5Br)\) is also less reactive than methyl iodide because the bromine group is less electronegative than the iodine group.

Here is a table showing the relative reactivity of the alkyl halides in the answer choices: