Which feature of blockchain ensures the history of transactions cannot be altered?

• A. Decentralized
• B. Immutable
• C. Verifiable
• D. Centralized

Answer: (B)

Immutability is the ability of the ledger’s history to remain unchanged once recorded. In blockchain, each block contains a cryptographic hash of the previous block, creating a chain. If you try to alter any transaction, the hash of that block changes and the chain breaks, which auditors and nodes would detect. To make the altered history consistent again, you’d need to rework that block and every subsequent block and then win over the network’s consensus, which is practically infeasible in a large, well-secured blockchain. The decentralized nature helps distribution, but it’s the cryptographic linkage plus consensus that actually protects the history from tampering. Verifiability is about checking correctness, and decentralization is about distribution; neither by itself guarantees that the history cannot be altered, whereas immutability directly addresses unchangeable history.