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

Defense in depth is a security architecture strategy that implements multiple overlapping layers of controls - physical, technical, and administrative - so that if any single control fails, additional layers continue to provide protection. It intentionally uses diverse and varied mechanisms to eliminate single points of failure and increase the cost and complexity of a successful attack.

Covert channels are hidden or unauthorized communication paths used to leak information outside normal security controls, making them an attack vector rather than a defensive strategy.

Exponential backoff is a network retry algorithm that progressively increases wait time between retries to reduce congestion, and has no relation to layered security strategy.

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