Identifying constitutional isomers of secondary alcohols with formula C4H10O Steps:

• Secondary alcohols have OH attached to a carbon bonded to two other carbons.
• Possible skeletons for C4H10O: straight-chain butane and branched 2-methylpropane.
• In butane, OH at position 2 (equivalent to 3) gives CH3CH(OH)CH2CH3, butan-2-ol.
• In 2-methylpropane, the branch carbon is tertiary; OH there gives (CH3)3COH, a tertiary alcohol, not secondary. Why A is correct:
• Only the straight-chain skeleton allows a carbon with exactly two carbon attachments for the OH group, per the definition of secondary alcohols. Why the others are wrong:
• B: Branched skeleton yields no additional secondary alcohol, so not two.
• C: Confuses with total alcohol isomers (four) or larger formulas like C5H12O.
• D: Applies to all alcohol types, not just secondary.

Final answer: A