Identify the type of structural isomerism
Isomers are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. Constitutional or structural isomers are compounds with the same molecular formula but their structural formulas are different. They also called chain isomers because of the possibility of branching in carbon chains (e.g., n-butane and 2-methylpropane). Note that just the twisted versions of the original molecule can be mistaken by novices as structural or chain isomers (e.g., conformers).

Positional isomers: Positional isomers are constitutional isomers that have the same carbon skeleton and the same functional groups. They differ from each other in the location of the functional groups on or in the carbon chain. In other words, in position isomerism, the basic carbon skeleton of isomers remains unchanged, but important groups on the skeleton are moved around on it (e.g., 1-bromopentane, 2-bromopentane, and 3-bromopentane).

Functional isomers: Functional isomers are constitutional isomers bearing different functional groups (e.g., propanoic acid, methyl ethanoate, and 3-hydroxypropanal).

Accordingly, the answer given is correct: These are functional isomers, since each have at least one different functional group changed (keto vs aldehyde).