What is proof$-$of$-$work?

Arequirement to define an expensive computer calculation, also called mining, that needs to be performed in order to create a new group of trustless transactions $($blocks$)$ on the distributed ledger or blockchain.

A way to validate transactions and achieve a distributed consensus.

A type of digital currency in which a record of transactions is maintained, and new units of currency are generated by the computational solution of mathematical problems and which operates independently of a central bank.

All of these choices are correct.