Help the exhausted babysitter explain the situation to the parents.

The twins were completely silent, but ___ boys had chocolate on their faces, so I knew they were guilty!

The correct answer is both.

We use both directly before a plural noun (like "boys") to talk about two people or things together.

"Either" and "neither" must be followed by a singular noun (e.g., "either boy"). "Both of" is incorrect here because it requires a determiner like "the" or a pronoun (e.g., "both of the boys" or "both of them").