"You shouldn't trust them."
Translation:Tu ne devrais pas leur faire confiance.
There are stressed pronouns (moi, toi, lui...) that you would use in a sentence with a preposition to put emphasis on that particular part of the sentence:
Tu ne devrais pas leur faire confiance à eux.
But to use it you still need to put the indirect pronoun (leur) in the sentence at the right place. In this exercise, there's no need for the stressed pronoun.