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

Metamers are isomers that have the same molecular formula and belong to the same functional group, but the alkyl groups attached to the functional group are different.

To identify the metamer pair, examine each option:

1. CH₃CH₂CH₂CH₂OH (1‑butanol) vs CH₃−CH(OH)−CH₃ (2‑propanol). Both are alcohols, but they have different carbon numbers (C₄ vs C₃). They are not isomers of the same formula, so they cannot be metamers.

2. CH₃OCH₂CH₂CH₃ (methyl propyl ether) vs CH₃CH₂OCH₂CH₃ (diethyl ether). Both contain the ether functional group (‑O‑) and have the same molecular formula C₄H₁₀O. In the first molecule the ether oxygen is attached to a methyl group and a propyl group, while in the second it is attached to two ethyl groups. The alkyl groups on the oxygen are different, satisfying the definition of metamers.

3. CH₃COCH₃ (acetone) vs CH₃CH₂CHO (propanal). One is a ketone, the other an aldehyde; they belong to different functional groups, so they cannot be metamers.

4. CH₃CH₂CH₂CH₃ (n‑pentane) vs (CH₃)₂CHCH₃ (2‑methylbutane). Both are alkanes, but they have the same molecular formula C₅H₁₂. Since alkanes have no functional group other than the C–C backbone, the concept of metamers does not apply.

Only option 2 meets all the criteria for metamers: same formula, same functional group (ether), and different alkyl substituents on the functional group.

Therefore, the correct pair of metamers is CH₃OCH₂CH₂CH₃ and CH₃CH₂OCH₂CH₃.