Which haloalkane is MOST reactive toward $\mathrm{S_N 2}$ substitution (with a typical nucleophile like $\mathrm{OH^-}$)?

1. SN2 mechanism: ONE-STEP backside attack of the nucleophile on the carbon, with simultaneous expulsion of the leaving group. Transition state has 5 substituents around the central C. 2. STERIC FACTORS dominate: smaller alkyl groups around the central C → less crowded transition state → faster SN2. 3. Reactivity order for SN2: methyl > 1° > 2° > 3° (steric crowding by bulky groups around the central C SLOWS SN2). 4. Of the given choices, $\mathrm{CH_3Cl}$ has only HYDROGENS around the central C — least crowded, fastest SN2. 5. Tertiary $\mathrm{(CH_3)_3CCl}$ is the SLOWEST for SN2 (option B), but it's the FASTEST for SN1. 6. The remaining options fall between these extremes. _Source: NCERT Class 12 Chemistry Part 2, Ch 6, §6.5.1 (SN2 mechanism — steric effects on rate), p. 11–12._