The meaning of the third sentence of the final paragraph would remain most nearly the same if "regime" were replaced ...

Hi In the context of the passage, and to a large extent generally speaking, I feel that the words, "law," "principle," and "constraint" are more or less synonymous, so I thought how can one be correct and the other be incorrect when all three are mostly synonymous, and for this reason I proceeded to eliminate all three without giving it a second thought. But when I am reviewing this question, I feel that I got lucky! Did I get lucky? I probably should not have used that approach, which is eliminating answer-choices with largely overlapping meaning because in such case there is no justification for selecting one over the other when they all mean the same. Is there textual support for "system"? Thank you Mazen