What is blockchain?

• A. A chain of blocks linked by cryptographic hashes
• B. A single central ledger
• C. A database of user credentials
• D. A protocol for email encryption

Answer: (A)

Blockchain is a chain of blocks linked by cryptographic hashes that forms a distributed ledger. Each block contains a batch of transactions and the hash of the previous block, creating a tamper-evident sequence. If any data in a block changes, its hash changes and the chain integrity is broken, making tampering evident. Because the ledger is typically replicated across many independent nodes, consensus mechanisms ensure agreement on the order of transactions without a central authority, enhancing transparency and fault tolerance.

This helps distinguish blockchain from a single central ledger, which implies centralized control; from a simple database of user credentials, which is just data storage without the chained, tamper-evident structure; and from a protocol for email encryption, which addresses a different security goal altogether.