Once a record has been added to a Blockchain it can be altered.

The statement is false. A fundamental and defining characteristic of blockchain technology is immutability. Once a record (or transaction) is validated by the network and added to a block, and that block is appended to the chain, it cannot be altered or deleted. This is achieved through cryptographic hashing. Each block contains the hash of the preceding block, creating a secure and interdependent chain. Any attempt to change the data in a previously recorded block would alter its hash, which would in turn invalidate the hash link to the subsequent block and all blocks that follow it. This discrepancy would be immediately rejected by the network's consensus mechanism, thus preserving the integrity and permanence of the ledger.

The "True" option is incorrect because it contradicts the core security principle and value proposition of blockchain. The entire architecture, combining cryptographic linkage, distributed consensus, and transparency, is designed specifically to create a tamper-evident and unchangeable record of transactions. While theoretically possible through a 51% attack, it is computationally and economically infeasible in a robust, decentralized network, making the ledger effectively immutable.

1. National Institute of Standards and Technology (NIST), NISTIR 8202: Blockchain Technology Overview, (October 2018).

Section 2.1, Core Components: "Blockchains are immutable ledgers. This means that once a block has been added to the chain, it cannot be changed. If a change is needed, a new block must be added to the chain to reflect the change. The original block cannot be modified." (Page 7)

2. Narayanan, A., Bonneau, J., Felten, E., Miller, A., & Goldfeder, S. (2016). Bitcoin and Cryptocurrency Technologies: A Comprehensive Introduction. Princeton University Press.

Chapter 1, Section 1.3, How Bitcoin Achieves Decentralization: The text explains that the block chain functions as a "tamper-evident log." It states, "In short, the block chain ensures that all transactions are recorded in a global, tamper-evident, and publicly visible log." (Page 6)

3. University of California, Berkeley, CS 294-144: Blockchain, Cryptoeconomics, and the Future of Technology, Business, and Law.

Lecture 2: How Bitcoin Works: The course material explains that changing a block requires re-doing the proof-of-work for that block and all subsequent blocks. "To change history, an attacker must re-do the proof-of-work for the block they want to change AND all subsequent blocks... This makes the blockchain 'immutable'." This concept is a foundational element taught in the course.