What is a zero-knowledge proof (ZKP) and what core property makes it useful in cryptography?

A A proof technique requiring the verifier to already know the secret before verification can begin B A protocol allowing one party (the prover) to convince another (the verifier) that a statement is true without revealing any information beyond the truth of the statement itself C A method for proving that a cryptographic key has zero entropy and is therefore insecure D A compression algorithm that reduces a proof to zero bytes for efficient transmission