Which of the following is a cryptographic approach employed by many cryptographic algorithms and cryptosystems?

Symmetric-key cryptography uses a single shared key and, while widely used for bulk data encryption, does not address the key distribution problem and is not the single foundational approach employed across the broadest range of cryptosystems.

Public-key cryptography uses mathematically linked key pairs (public and private) and underpins protocols such as TLS, SSH, PGP, and certificate authorities at massive scale. It solves the key distribution problem inherent in symmetric systems and is employed in digital signatures, key exchange, and encryption across countless standards. Its broad adoption across the widest range of cryptosystems makes it the most widely employed cryptographic approach referenced in the question.

Asymmetric-key cryptography is technically synonymous with public-key cryptography, but 'public-key cryptography' is the standardized term used in formal cryptographic literature and NIST standards to describe this approach.

Quantum cryptography is an emerging field leveraging quantum mechanical properties for key distribution and has not yet been broadly employed across many existing cryptographic algorithms and cryptosystems.