"He would have bought us something to drink."
Translation:Il nous aurait offert à boire.
Something is definitely wrong - Earlier on in the test it insists on saying" Il nous aurait apporte a boire" - now when it's asking us to translate it back into french, it's insisting on using "offert" or "paye" and to top it off, bought is the past of buy, not bring, so it should be "brought us something to drink" otherwise surely you would be using the verb acheter!
No, because if it were "Il nous aurait achetés...", you would be agreeing the past participle to "us", and that would mean that "us" is the thing bought by him (instead of "something to drink"). And it would not make sense anyways, since "quelque chose à boire" would have no function in the sentence.
In grammar terms, you only agree the past participle with the direct object (and never the indirect object, like in the sentence showed here), and that may only occur in two situations: 1> with the auxiliary "être" when the subject and the direct object are the same; 2> or with the auxiliary "avoir" when the past participle appears in the sentence after the direct object (or some word/words representing the direct object, for instance a "pronom complément").
In fact, the "nous" in the sentence above was what made you confused here. That's because "nous" is a word that remains the same no matter the several diverse functions it may assume: pronom sujet, pronom réfléchi, pronom réciproque, pronom tonique, pronom complément d'object direct (COD), pronom complément d'objet indirect (COI).
So, "nous" is indeed sometimes in the fonction of a "pronom complément d'objet direct" (COD), but that is not the case here. Here "nous" is the "pronom complément d'objet indirect" (COI).