What type of malware continually changes to evade signature-based detection?

Polymorphic viruses are designed to evade detection by constantly changing their signature, which is the unique pattern that security software looks for to identify malware. This capability allows them to alter their code each time they infect a new system, making it much more challenging for traditional antivirus programs that rely primarily on signature-based detection methods. This adaptability ensures that they can bypass security measures that are not equipped to recognize their ever-changing forms.

In contrast, the other types of malware listed do not possess this specific characteristic. Trojans often disguise themselves as legitimate software but do not change their code to evade detection. Worms self-replicate across networks but typically do not alter their payloads on the fly. Botnets refer to a network of compromised devices controlled by malware but do not inherently change their code to avoid detection. Thus, the defining trait of polymorphic viruses as it relates to constantly changing to evade detection makes them the correct choice for this question.