Which security strategy requires using several, varying methods to protect IT systems against attacks?

Defense in depth applies several distinct security mechanisms across different layers - physical, network, host, application, and data - so no single point of failure exposes the entire system. The use of varying methods ensures that an attacker who bypasses one control still faces additional barriers. This multi-layered approach directly matches the question's description of 'several, varying methods.'

The three-way handshake is a TCP connection establishment process (SYN, SYN-ACK, ACK), not a security protection strategy.

Covert channels are hidden communication paths used to exfiltrate data or bypass controls, making them an attack vector rather than a defensive strategy.

The exponential backoff algorithm is a retry timing mechanism used in networking and distributed systems, not a security defense strategy.