We see that the sentence says that "an infant’s reactions to its first stressful experiences are part of a natural process of development" and that they are NOT "___________ signs of adolescent anxiety."
So, the word that fills the blank must express something that the reactions are not if they are "part of a natural process of development."
(B) normal is in line with "part of a natural process of development." After all, something normal would be part of a natural process of development. So, normal is not something the reactions would not be.
(C) monotonous (dull and repetitious) isn't something the reactions would clearly not be. Something that's part of a natural process could be dull and repetitious.
(D) virtual (existing in essence but not in fact) doesn't fit the scenario at all. There's no reason to believe that the reactions are or are not virtual.
(E) typical (usual for a particular person or thing) is similar to "normal." So, typical is also in line with "part of a natural process of development." After all, something typical of an infant would likely be part of a natural process of development. So, typical is not something the reactions would not be.
Notice that, because "normal" and "typical" are in line with "part of a natural process of development," these choices are tricky. If we don't see exactly what's going on in the sentence, we can choose "normal" or "typical" because they seem to fit the topic.
(A) prophetic (predicting the future) makes sense because it makes sense that reactions that are "part of a natural process of development" would NOT be signs predicting adolescent anxiety, which is presumably not usual for a growing child.
The correct answer is (A).