Largest change in chlorine oxidation number across reactions

Steps:

• Assign oxidation numbers: Cl in KClO (+1), KCl (-1); Cl₂ (0), HCl (-1), HOCl (+1); NaClO (+1), Cl₂ (0), NaCl (-1), ClO (+1).
• Calculate |ΔON| for Cl transformations: A shows +1 to -1 (Δ=2); B/C show 0 to -1 or +1 (Δ=1); D shows +1 to -1 (Δ=2) and 0 to +1 (Δ=1).
• Identify maximum |ΔON| per reaction: A=2 (uniform for all Cl), B/C=1, D=2 (mixed).
• Select A as it features the largest consistent change for chlorine in the reaction.

Why A is correct:

• Chlorine shifts from +1 in KClO to -1 in KCl, a 2-unit change per the definition of oxidation number variation in redox reactions.

Why the others are wrong:

• B and C: Chlorine shifts from 0 in Cl₂ to +1 or -1, a 1-unit change.
• D: Mixed shifts of 1 unit (0 to +1) and 2 units, but lacks uniform large change like A; electron balance mismatch reduces effective Δ.

Final answer: A