What is the pronoun of THAT in These are documents will be most useful?

The pronoun of "THAT" in "These are documents that will be most useful" is those.

Here's why:

* "That" is a demonstrative adjective in this sentence, modifying the noun "documents".

* To use a pronoun in place of "those documents", you need a pronoun that refers to plural nouns.

* "Those" is the correct plural pronoun to replace "that documents".

The sentence would then become: "These are documents those will be most useful."