After eliminating unidiomatic choices involving the use of the verb ‘consider’, we are left with A and D. The gist of the topic is that the Robert stunned his family, friends, and neighbors, by releasing all (note
all) the slaves he held in his enrollment. The is the implication of the use of the definite article ‘the‘in the original text. When you drop the definite article, then it means that Robert had more than 500 slaves and of them he released only 500. This is distorted intention. Hence A is the right choice
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