Explanation:

'Would let transacting parties remain' is the correct answer. 'Would' is used to preserve the future aspect when talking about the past. If you are writing about past events, you can use it to indicate something that was in the future at that point in time but is not necessarily in the future right now. 'Could' represents past of 'can' and when you want to convey something politely. Here, the future aspect of Bitcoin is being discussed, hence would is a more suitable response. Would letting is grammatically incorrect; hence option A can be eliminated. Option 4 makes the sentence grammatically incorrect; hence it can be eliminated