Final answer:
Bitcoin is the most popular and fastest-growing digital currency, followed by Ethereum, Ripple (XRP), and Litecoin.
Step-by-step explanation:
Bitcoin is the most popular and fastest-growing digital currency. It is considered the most widely accepted and used cryptocurrency as a medium of exchange for goods and services. Other popular cryptocurrencies include Ethereum, Ripple (XRP), and Litecoin. However, Bitcoin remains the most dominant and influential cryptocurrency in terms of popularity and growth.