The pair of molecules that are metamers among the following is :

(D) \(CH _3 OCH _2 CH _2 CH _3\) and \(CH _3 CH _2 OCH _2 CH _3\)
Metamerism is a type of structural isomerism where isomers have the same molecular formula but differ in the nature of the alkyl groups attached to the same polyvalent functional group (such as \(- O -,- S -,- NH -,- CO -\) ).
In option (1), \(CH _3 CH _2 CH _2 OH\) and \(CH _3- CH ( OH )- CH _3\) are position isomers because the position of the - OH group differs.
In option (2), \(CH _3 CH _2 CH _2 CH _2 CH _3\) and \(\left( CH _3\right)_2 CHCH _2 CH _3\) are chain isomers because the carbon skeleton differs.
In option (3), \(H _3 C - CO - CH _3\) and \(H _3 C - CH _2- CHO\) are functional isomers because they have different functional groups (ketone and aldehyde).
In option (4), \(CH _3 OCH _2 CH _2 CH _3\) (methoxypropane) and \(CH _3 CH _2 OCH _2 CH _3\) (ethoxyethane) have the same polyvalent functional group ( \(- O -\) ) but different alkyl groups attached to it (methyl and propyl versus two ethyl groups). Thus, they are metamers.