Alcohols are isomers if they share the same molecular formula but differ in structure

Steps:

• Determine molecular formulas for each alcohol: count carbons and hydrogens, adding OH.
• Butan-1-ol: CH₃CH₂CH₂CH₂OH (C₄H₁₀O); 2-methylpropan-1-ol: (CH₃)₂CHCH₂OH (C₄H₁₀O).
• Pentan-1-ol: CH₃(CH₂)₃CH₂OH (C₅H₁₂O); propan-1-ol: CH₃CH₂CH₂OH (C₃H₈O); propan-2-ol: CH₃CH(OH)CH₃ (C₃H₈O); 2-methylpropan-2-ol: (CH₃)₃COH (C₄H₁₀O).
• Compare pairs: only option A matches formulas (both C₄H₁₀O).

Why A is correct:

• Both have formula C₄H₁₀O but different carbon chains: straight vs. branched.

Why the others are wrong:

• B: Pentan-1-ol (C₅H₁₂O) vs. 2-methylpropan-1-ol (C₄H₁₀O); different carbon count.
• C: Propan-1-ol (C₃H₈O) vs. 2-methylpropan-1-ol (C₄H₁₀O); different carbon count.
• D: Propan-2-ol (C₃H₈O) vs. 2-methylpropan-2-ol (C₄H₁₀O); different carbon count.

Final answer: A