Spot the error.The couples have been so close that they even lived next door to each other themselves for a while.

Explanation

The correct answer is Themselves because in this context, the reflexive pronoun 'themselves' is unnecessary. The phrase 'next door to each other' adequately expresses the mutual proximity of the couples, making 'themselves' redundant. The other options are correct in their grammatical form and do not contain errors: 'Have been' appropriately indicates a state of being, 'Even' effectively emphasizes the situation, and 'Each' correctly refers to the couple members.