Which threat involves software being used to gain unauthorized access to a computer system?

A "ping of death" is an old denial-of-service attack involving oversized ICMP packets, not software designed to gain unauthorized access.

NTP amplification is a denial-of-service (DoS) attack that leverages NTP servers to flood a target with amplified traffic, not software for unauthorized system access.

An HTTP flood is a denial-of-service (DoS) attack where a large volume of HTTP requests overwhelm a web server, not software for gaining unauthorized access.

A virus is a malicious computer program that, when executed, replicates itself by modifying other computer programs and inserting its own code. Beyond replication, viruses often carry payloads designed to perform various malicious actions, including gaining unauthorized access, stealing data, or disrupting system operations.