Which of the following is a method of gaining access to a system that bypasses normal authentication?

Teardrop is a denial-of-service attack that sends malformed, overlapping IP fragments to crash or destabilize the target system - it does not bypass authentication.

A Trojan horse is malware disguised as legitimate software to trick users into executing it - it does not itself bypass authentication mechanisms.

A back door is a hidden entry point intentionally or maliciously placed in a system that circumvents standard login and authentication procedures. Unlike other attack types, its defining characteristic is allowing unauthorized access without triggering normal credential checks. It may be installed by an attacker after initial compromise or embedded by a malicious insider.

Smurf is an ICMP-based distributed denial-of-service amplification attack that floods a victim with traffic - it has no authentication bypass capability.