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

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 $-\text{O}-$, $-\text{S}-$, $-\text{NH}-$, $-\text{CO}-$).

In option (1), $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ and $\text{CH}_3-\text{CH(OH)}-\text{CH}_3$ are position isomers because the position of the $-\text{OH}$ group differs.

In option (2), $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_3$ and $(\text{CH}_3)_2\text{CHCH}_2\text{CH}_3$ are chain isomers because the carbon skeleton differs.

In option (3), $\text{H}_3\text{C}-\text{CO}-\text{CH}_3$ and $\text{H}_3\text{C}-\text{CH}_2-\text{CHO}$ are functional isomers because they have different functional groups (ketone and aldehyde).

In option (4), $\text{CH}_3\text{OCH}_2\text{CH}_2\text{CH}_3$ (methoxypropane)$and \text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3$ (ethoxyethane) have the same polyvalent functional group ($-\text{O}-$) but different alkyl groups attached to it (methyl and propyl versus two ethyl groups). Thus, they are metamers.

Answer: $\text{CH}_3\text{OCH}_2\text{CH}_2\text{CH}_3$ and $\text{CH}_3\text{CH}_2\text{OCH}_2\text{CH}_3$