How many bitcoin are available?

Bitcoin has a fixed supply limit of21 millioncoins, as defined by its protocol. This cap is coded into Bitcoin's original protocol and cannot be altered without a consensus change, ensuring its scarcity and value proposition as a "digital gold."

Key Details:

Fixed Supply: Bitcoin's supply limit is integral to its design, as it imposes a hard cap on the total number of coins that can ever be mined. This limited supply is one of the factors that gives Bitcoin its deflationary nature and potential as a store of value.

Mining Schedule: New Bitcoins are released into circulation through mining rewards, which halve approximately every four years (in an event known as the "halving"). This gradually reduces the issuance rate until the maximum supply is reached around the year 2140.

Difference from Other Cryptocurrencies: Unlike some cryptocurrencies that have flexible or inflationary supply models, Bitcoin’s finite supply is a distinguishing feature that appeals to those concerned with long-term value preservation.

Therefore,D. 21 millionis the correct answer, as it accurately reflects the total number of Bitcoin that will ever exist.