I think it is due to fear for prisoners and want for power for guards
Personally, I'd go with D. heartlessness.
It's not A, fitness, because we don't know if Mr. Gray was fit - we only know he was tall and slender; B is also wrong because the text says nothing about him being spiteful; C is wrong for the same reason. Thus, this leaves us with D, because the text says he is 'unsentimental,' which means the same as heartless.
Answer:
B
Explanation:
dependent clauses usually don't make sense without context.