19) The inhabitants of an island are divided into two categories: those who are always truthful and those who always lie. Among three inhabitants of the island, Andrea, Barbara, and Ciro, the following conversation takes place: Andrea says: "Barbara is truthful", Barbara says: "Andrea and Ciro are truthful", Ciro says: "Andrea is a liar". We can conclude that

(A) they are all three truthful, (B) they are all three liars, (C) Andrea and Barbara are truthful and Ciro is a liar, (D) Andrea and Barbara are liars and Ciro is truthful, (E) Andrea is truthful and Ciro and Barbara are liars.
19. The answer is $(\mathbf{D})$.

Suppose Andrea is honest. Then the statement he makes is true, and therefore it is true that Barbara is honest. Consequently, it is also true what Barbara says, namely that both Andrea and Ciro are honest. In particular, she affirms that Ciro's statement is true, which is that Andrea is a liar. This contradicts our initial assumption. Therefore, Andrea cannot be honest, which means Andrea is a liar.

As a consequence, Andrea's statement is false, meaning that Barbara is also a liar.

On the other hand, Barbara's statement cannot be true (since Andrea is a liar, Andrea and Ciro cannot both be honest). At this point, nothing can be said about Ciro's situation. However, we still need to examine the statement he made, which is "Andrea is a liar," and we know this to be true. Therefore, Ciro is honest.