The answer should be A.
A. "One hallmark of the mortgage-banking crisis, robo-signing, in which banks..." - This option introduces "robo-signing" as a noun phrase describing one aspect of the crisis. It's clear and grammatically sound.
B. "Robo-signing was one hallmark of the mortgage-banking crisis, in which banks..." - This option uses "robo-signing" as the subject of the sentence, followed by the phrase "in which banks." While it's clear, it might sound a bit awkward due to the separation between "robo-signing" and the context related to banks.
C. "Robo-signing, which was one hallmark of the mortgage-banking crisis and which banks..." - This option uses "which" twice, creating redundancy. It can also be seen as less concise compared to other options.
D. "One hallmark of the mortgage-banking crisis was robo-signing, in which banks..." - This choice uses the word "was" incorrectly, creating a fragment with the use of the phrase "has led to" in the last clause of the paragraph. It is grammatically incorrect.
E. "One hallmark of the mortgage-banking crisis was robo-signing, and banks" - This option is incomplete. It lacks a direct continuation or action following "banks," making it an incomplete sentence.