Explanation

The relative pronoun used after 'such' is 'as', not 'that' — the standard correlative construction is 'such … as', e.g. 'such friends as you can trust'. The clause 'that you can trust' therefore violates the rule, so the error lies in part (C). Parts (A) and (B) are grammatically sound: 'Choose only such friends' is a well-formed imperative phrase. Hence the error is in (C), not in the rest of the sentence.