Which mechanism adds security and immutability to blockchain technology?

The mechanism that adds security and immutability to blockchain technology is cryptographic hashing. In a blockchain, each block contains a unique hash that is derived from the data within that block and the hash of the previous block. This process creates a chain of blocks, where altering any information in a previous block would change its hash, thus invalidating not only that block but all subsequent blocks as well. This cryptographic principle ensures that once a block is added to the blockchain, it cannot be modified without detection, which establishes a high level of security and immutability.

While other options such as Proof of Stake, consensus algorithms, and smart contracts contribute to the overall functionality and efficiency of blockchain systems, they do not provide the same foundational level of security and tamper-evident properties that cryptographic hashing offers. Consensus algorithms, for example, help maintain agreement among distributed nodes but do not inherently secure data; likewise, smart contracts provide programmable conditions for executing transactions but are built on the existing secure framework provided by cryptographic hashing. Proof of Stake is a method of reaching consensus but does not impact the core immutability of stored data in the same way that cryptographic hashing does.