In a blockchain, the structure that summarises all the transactions in a block into a single hash is called

The correct answer is B, Satoshi Nakamoto. The paper titled Bitcoin: A Peer-to-Peer Electronic Cash System was circulated in 2008 under the name Satoshi Nakamoto, a pseudonym whose owner has never been established, and it set out how a chain of hashed blocks and proof of work could prevent double spending without any trusted third party. The first block of the chain was mined in January 2009. Option A is wrong because Vitalik Buterin proposed Ethereum in 2013, five years later, and Ethereum is a different network built for smart contracts. Option C is wrong because Nick Szabo coined the term smart contract in the 1990s and designed an earlier scheme called bit gold, but did not write this paper. Option D is wrong because Ralph Merkle devised the Merkle tree, the hashing structure that bitcoin uses inside a block, decades before bitcoin existed.

The correct answer is B, SHA-256. Bitcoin uses the SHA-256 hash function, part of the Secure Hash Algorithm family, which turns an input of any length into a fixed output of two hundred and fifty-six bits. The function is one-way, so the input cannot be derived from the output, and it is collision resistant, so two different inputs practically never give the same hash; mining consists of searching for a nonce that makes the block hash fall below a target value. Option A is wrong because MD5 is an older and now broken hash function, no longer considered secure. Option C is wrong because AES is a symmetric encryption cipher, not a hash function, and encryption is reversible with the key while a hash is not. Option D is wrong because RSA is a public key encryption and signature algorithm; bitcoin signs transactions with elliptic curve cryptography, not RSA.

The correct answer is B, twenty-one million. The bitcoin protocol fixes the total supply at twenty-one million coins. New coins enter circulation only as the reward paid to whoever adds a block, and that reward is halved every two hundred and ten thousand blocks, which works out to roughly every four years, so issuance falls geometrically towards zero. This built-in scarcity is why bitcoin is often described as deflationary by design. Option A is wrong because one crore, that is ten million, is below the protocol limit. Option C is wrong because one hundred million is the number of the smallest units, called satoshis, in a single bitcoin, not the number of bitcoins. Option D is wrong because an unlimited supply is the feature of ordinary fiat currency, which a central bank can issue at will, and the fixed cap is precisely the contrast bitcoin was designed to draw.

The correct answer is C, Proof of Work. In proof of work, the computers called miners compete to find a nonce that makes the hash of the proposed block meet a difficulty target. The search requires enormous numbers of trials and therefore real electricity, which is the cost that makes rewriting the chain uneconomic; the winner adds the block and receives the block reward and the fees. Option A is wrong because proof of stake, in which validators are chosen in proportion to the coins they lock up, is used by Ethereum after its change of mechanism in 2022 and by many newer chains, but not by bitcoin. Option B is wrong because proof of authority relies on a small set of identified validators and suits private or consortium chains. Option D is wrong because practical Byzantine fault tolerance is a voting-based mechanism used in permissioned systems such as Hyperledger Fabric.

The correct answer is A, Vitalik Buterin. Ethereum was described by Vitalik Buterin in a white paper of 2013 and the network went live in 2015. Its contribution was to add a general purpose computing layer, the Ethereum Virtual Machine, on top of a blockchain, so that programs called smart contracts could be stored on the chain and run by themselves; contracts are written in languages such as Solidity and the computation is paid for in a fee called gas. Its token is ether. Option B is wrong because Satoshi Nakamoto is the name on the bitcoin paper of 2008, and bitcoin has no general contract layer. Option C is wrong because Charlie Lee created Litecoin, an early alternative coin based on the bitcoin code. Option D is wrong because Hal Finney was an early bitcoin developer who received one of the first bitcoin transactions, not the founder of Ethereum.