According to the context of the sentence, the word that best fits is "high" as it is speaking of something dangerous.Also, "high" is the best alternative to modify the noun "risk" taking into account the meaning as a whole.
in this case, if somenthing is dangerous, it cannot be "Low" or "medium" risk so options C and D are not suitable.
As regards the adjective "negative" it does not "collocate" with the noun "risk".
The answer is D. Harlow’s monkey
experiments involved separating infant monkeys from their mothers a couple of
hours after birth and raising them with mechanical maternal figures. He
repeatedly made comments that the behavior of the subjects mirrored that of children
being observed in institutions.
