Day 51 Answer:

C) A peer-to-peer timestamp server

In the Bitcoin White Paper, Satoshi proposes a peer-to-peer timestamp server that uses proof-of-work to record the chronological order of transactions. This ensures that once a block is added to the chain, it becomes increasingly impractical to modify past transactions, preventing double-spending without relying on a trusted third party. nostr:note19jlh07q97g94ktrl28u69am9ghmg86pmnghtl2zkejyjwn828mxsptdtfq