The multiple choice text offers the alternatives băiatului and băieților. In my opinion both make a correct sentence; I can give a single schoolbag to several boys. It would be better if the “wrong” alternative had a grammatical problem (such as a nominative noun).
If the multiple choice still keeps the unstressed form of the pronoun (îi), then using the plural is indeed wrong. It should be "Le dau băieților un ghiozdan roșu."