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 (C=O in the middle of a carbon - chain), the second is an alcohol (has an - OH group), the third is an ester (has a - COO - group), and the fourth structure has an oxygen atom bonded between two carbon - containing groups in a ring, which is an ether (a cyclic ether specifically, tetrahydrofuran).

Answer:

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