Usually information security in a network is achieved by :

Information (network) security means protecting data as it travels across a network so it stays confidential, unaltered in transit against tampering (integrity), and traceable to a genuine sender who cannot later deny sending it (authentication and non-repudiation). Cryptography is the general-purpose technique that provides these guarantees: encryption converts readable data into ciphertext for confidentiality; message-authentication codes and signed/authenticated hashes detect adversarial tampering for integrity; and digital signatures establish authentication together with non-repudiation, since only the holder of a private signing key could have produced a valid signature.

Among the given choices, only cryptography is itself a security technique; the mechanism that achieves information security in a network is therefore cryptography.

• Layering — an architectural principle for organizing protocol-stack functions into levels (e.g., OSI/TCP-IP); it describes network structure, not a security mechanism.

• Grade of service — a traffic/QoS measure of blocking or delay probability under a given load; it concerns performance and capacity, not protection of data.

• None of these — applies only when none of the other listed terms names the mechanism; cryptography does name it, so this option does not apply.

This is consistent with standard networking and security references — for example, NIST’s glossary describes cryptography as the discipline embodying principles and means for protecting information — and with the published answer key for this item (UGC NET Computer Science Paper II, June 2009, question 31).