Which of the following is a common type of malware?

A virus is a common type of malware specifically designed to replicate itself by attaching to other programs or files. Once it infects a host, it can spread to other devices and systems, often causing various forms of damage, including data corruption, loss of system functionality, and unauthorized access to sensitive information.

Viruses typically operate by exploiting weaknesses in software or leveraging social engineering tactics to persuade users to execute them. They can take many forms, such as file-infector viruses, macro viruses, and polymorphic viruses, making them versatile threats in the cybersecurity landscape.

In contrast, the other options represent technologies or services rather than malware. Routers and ISPs (Internet Service Providers) serve networking functions and facilitate internet access, while firewalls are security devices designed to monitor and control incoming and outgoing network traffic based on predetermined security rules. None of these options fall under the category of malware, which fundamentally emphasizes harmful software designed to infiltrate and damage systems.