Which of the following best describes the immutability of a blockchain?

The immutability of a blockchain is best described by stating that data, once written, cannot be altered. This characteristic is a fundamental aspect of blockchain technology. When a transaction is recorded onto the blockchain, it is added to a block that is cryptographically linked to the previous block. This process creates a secure chain of blocks where each block is immutable, meaning it cannot be changed or deleted without altering all subsequent blocks and the consensus of the network.

This immutability ensures that the historical data is preserved and provides a level of integrity and trustworthiness to the information stored, as users can be confident that the data has not been tampered with after it has been verified and added to the blockchain. This feature is essential for applications that rely on accurate records, such as cryptocurrency transactions, supply chain management, and secure voting systems.

The other options do not accurately capture the essence of immutability in a blockchain context. For instance, altering data with consensus contradicts the notion of immutability, and stating that data is stored temporarily or can be deleted runs counter to the very definition of a blockchain's persistent and unchangeable nature.