I was also looking at both (A) and (C), but I chose (C) because it was a paraphrase of the text and required less inference.
Sometimes, when struggling between two seemingly similar answers, I find it helpful to look for direct paraphrases of the answer choices in the passage. In this case, there are several paraphrases of (C) but not (A). Let's find the paraphrases.
In line 44, it says, "To the English the word 'constitution' meant the whole body of law and legal custom formulated since the beginning of the kingdom"
(C) a cumulative corpus of legislation and legal traditions
"cumulative" = "formulated since the beginning of the kingdom" (note the word "since" implies that it was an ongoing process, not a single point in time, hence "cumulative")
"corpus of legislation" = "body of law"
"legal traditions" = "legal custom"
That's an almost perfect match.
How about (A)?
(A) the legal foundation of the kingdom
(A) is quite general, and I couldn't figure out exactly what "the legal foundation" meant. Does it refer to the power given to the king? Does it refer to laws, written or otherwise? I would need to decide what it means because it's not specified.
You'll probably notice that if we break apart the answer choices, the almost word-for-word paraphrase of (C) is provable, and requires no interpretation on our part, whereas (A) requires we make the leap of faith that the "legal foundation" is the same as what is stated in the text.
