One strong base for analyzing the answer is holding the point that john has not to be left alone, so either one of his parents has to be present or Jonas has to be present with him.
Let's jump on to the options
(A) Mr. Smith crosses the river first : Mr. smith can cross the river without violating that john is alone, as Mrs. smith is with John
(B) Mr. Smith crosses the river last.: Mrs. smith can cross the river without violating that john is alone, as Mr. Smith is with John
(C) Mr. and Mrs. Smith do not cross together : Both Mr. and Mrs. Smith can cross the river considering the fact that John is with Jonas.
(D) John crosses the river first.: Can't be true, if john crosses the river first, he has to stay on the opposite side of the river alone while Jonas has to bring the boat back to Mr. and Mrs. Smith
(E) John crosses the river second: Yes, possible. If john crosses 2nd, he wont be alone in either side. Doesn't go against the statement.