Acid-catalyzed ketone isomerization preserves empirical formula Steps:

• Identify compounds that react with HCl(aq): ethers cleave, diols rearrange/dehydrate, ketones enolize/isomerize.
• Check empirical formulas: all differ in products except where isomerization occurs without atom change.
• Test A (acetone, C3H6O): no isomeric ketone possible; no reaction yielding same formula.
• Test B (3-pentanone, C5H10O): isomerizes to 2-pentanone via common enol under acid; same C5H10O. Why B is correct:
• 3-Pentanone shares enol form CH3C(OH)=CHCH2CH3 with 2-pentanone, enabling HCl(aq)-catalyzed tautomerization to isomer with identical molecular/empirical formula C5H10O. Why the others are wrong:
• A: No positional isomer exists for propanone; remains unchanged.
• C: Cleavage gives C2H5Cl + C2H5OH (C4H11OCl total); different empirical.
• D: Rearrangement yields butan-2-one + H2O (C4H8O); loses H2O, changes to C2H4O empirical.

Final answer: B