KEAM 2012 Physics and Chemistry Paper

SN2 reactions involve the formation of intermediate transition state, thus less hindered alkyl halide readily undergoes SN 2 reaction.
Moreover, C−I bond is less stable than C−Br or C−Cl bond, thus alkyl iodides are more reactive as compared to other halides.
Hence, CH3CH2I is most reactive towards SN2 reaction.