Question

Each guide was helpful, but we forgot to thank _____. Which pronoun correctly completes the sentence? them him her you

Answer 1

Each guide was helpful, but we forgot to thank them.
You should use the plural pronoun, because we don't know whether the guides were only males, or females, or whether there were both sexes. 

Answer 2

Each guide was helpful, but we forgot to thank them.

The pronoun refers to each and every one of the guides. Since there are more than one guide we need to use a plural pronoun. Him, her, and you are all singular pronouns. Them is the only plural pronoun.