Unique C4H10O isomer with secondary OH on branched-like carbon

Steps:

• Identify four alcohol isomers for C4H10O: butan-1-ol (primary, straight), butan-2-ol (secondary, straight), 2-methylpropan-1-ol (primary, branched), 2-methylpropan-2-ol (tertiary, branched).
• Select for secondary alcohol: OH on carbon bonded to two other carbons; only butan-2-ol qualifies (CH3-CH(OH)-CH2CH3).
• Check branched carbon skeleton: butan-2-ol features a branch point at the secondary carbon, attached to methyl and ethyl groups.
• Confirm no others fit: branched isomers lack secondary OH position.

Why A is correct:

• Exactly one isomer, butan-2-ol, satisfies secondary alcohol on a carbon skeleton with branching via unequal alkyl groups (methyl + ethyl).

Why the others are wrong:

• B: Exceeds the single qualifying isomer.
• C: Overcounts; only one meets both criteria.
• D: Far too many; ignores restrictions.

Final answer: A