Solution:

The answer is $\mathbf{(C)}$. Certainly Luca cannot be truthful, since the two girls contradict each other, so they cannot both be telling the truth. If Maria were telling the truth, Nicola would be lying; therefore, it would not be true that there is a truthful girl, and in particular Maria would not be one, contradiction. Hence, Maria lies. So there is at least one boy who always tells the truth, and it must necessarily be Nicola. Therefore, by what he says, one of the two girls is truthful, and since it cannot be Maria, it is Paola. On the other hand, the latter states that there is only one boy who tells the truth, and from what was deduced earlier, this statement is true. Hence, the truthful ones are Nicola and Paola.