The correct option is D None of the above
None of the given compounds is an isomer of CH3CH2CH2CH2−OH. 1-butanol CH3CH2CH2CH2−OH has 4 C atoms whereas CH3CH2CH2−OH and CH3CH(OH)CH3 have 3C atoms only. Isomers have same molecular formula, i.e, they should have same number of C atoms. Similarly, CH3CH2CH2CH3 contains no O atom whereas CH3CH2CH2CH2−OH has one O atom.