A smart contract is

A$.$ a method for identifying items through the use of radio signals from a tag attached to an item.

B$.$ a codified agreement that self$-$executes when the conditions are met.

C$.$ a digital record of transactions in which individual records, called blocks, are linked together in a single list, called a chain.

D$.$ the practice of using a network of remote servers hosted on the Internet to store, manage, and process data.