Identify the pair of molecules that exhibit functional isomerism...

Functional isomerism arises when two compounds have the same molecular formula but different functional groups.

In option (A), $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ and $\text{CH}_3\text{CH}(\text{CH}_3)\text{CH}_2\text{OH}$ are both primary alcohols. They differ in the arrangement of the carbon chain, making them chain isomers.

In option (B), $\text{CH}_3\text{CH}_2\text{COOH}$ is a carboxylic acid and $\text{CH}_3\text{COOCH}_3$ is an ester. Both have the same molecular formula $\text{C}_3\text{H}_6\text{O}_2$ but possess different functional groups. Thus, they are functional isomers.

In option (C), $\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$ and $\text{CH}_3\text{CH}(\text{NH}_2)\text{CH}_3$ are both primary amines differing only in the position of the amino group, making them position isomers.

In option (D), $\text{CH}_3\text{CH}_2\text{SCH}_2\text{CH}_3$ and $\text{CH}_3\text{SCH}_2\text{CH}_2\text{CH}_3$ are both thioethers differing in the alkyl groups attached to the sulfur atom, making them metamers.

Answer: $\text{CH}_3\text{CH}_2\text{COOH}$ and $\text{CH}_3\text{COOCH}_3$