What characteristic defines a polymorphic virus?

A polymorphic virus is specifically defined by its ability to change its code to avoid detection by antivirus software. This characteristic allows it to generate different versions of itself each time it infects a system, ensuring that signature-based detection methods, which rely on recognizing known patterns in malicious code, are less effective. By continuously altering its code or using encryption, a polymorphic virus can persist in an environment where detection tools are employed.

The other characteristics provided in the options highlight different behaviors of malware but do not specifically define a polymorphic virus. For instance, while disguising itself with encryption is a tactic sometimes used by polymorphic viruses, the essence of a polymorphic virus lies in its ability to modify its code. The ability to spread through network connections pertains more generally to how many types of malware propagate, but is not a defining attribute of polymorphic viruses. Additionally, replication without a carrier is more characteristic of worms than viruses. Therefore, the defining trait of a polymorphic virus is its code-changing capability, making the correct choice that which acknowledges this function.