Question

which is an example of an ether? none of these structures is an ether.

Explanation:

An ether has the general formula $R - O - R'$, where $R$ and $R'$ are alkyl or aryl groups. The first structure is a ketone ($R - CO - R'$), the second is an alcohol (has - OH group), the third is an ester ($R - COO - R'$). The fourth structure is a cyclic ether (oxolane), which fits the ether - general formula with the oxygen atom bonded to two carbon - containing groups within the ring.

Answer:

The fourth structure (the cyclic compound with an oxygen in the ring)