Explanation

The sentence contrasts the infant’s reactions, part of a normal developmental process, with future unhappiness and anxiety. The missing word describes signs of adolescent anxiety as they relate to the infant. Choice A is correct: “prophetic” signs, like harbingers, foretell future occurrences, and for the infant, adolescent anxiety is a future occurrence. Since an infant cannot literally display signs of adolescent anxiety, “normal,” “monotonous,” and “typical” are all incorrect. And “virtual” is incorrect, because virtual signs are not real signs, and what Kagan is denying is that the infant’s reactionsare not real signs of later unhappiness.
Thus the correct answer is prophetic (Choice A).
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The sentence claims that stressful experiences are normal and NOT _______ signs of adolescent anxiety. So we want a word that is similar to early' or 'prescient'. Thus we choose prophetic.