Step 1: Remember what functional isomers require.
Two classes of compounds can be functional isomers of each other only if they can share exactly the same molecular formula, just with the atoms arranged into a different functional group.
Step 2: Write out the general formulas.
Alcohols and ethers both fit $\text{C}_n\text{H}_{2n+2}\text{O}$, so they can match. Aldehydes and ketones both fit $\text{C}_n\text{H}_{2n}\text{O}$, so they can match. Carboxylic acids and esters both fit $\text{C}_n\text{H}_{2n}\text{O}_2$, so they can match too.
Step 3: Now compare alcohol and ester formulas. \[ \text{Alcohol: } \text{C}_n\text{H}_{2n+2}\text{O} \quad \text{Ester: } \text{C}_n\text{H}_{2n}\text{O}_2 \] These two formulas differ both in the number of hydrogens and in the number of oxygens, for every possible value of $n$.
Step 4: Draw the conclusion.
Because their molecular formulas can never coincide, an alcohol and an ester can never be built from the same set of atoms, so they cannot be functional isomers. \[ \boxed{\text{Alcohol and esters}} \]