Which of the following best describes a virus?

A virus is best described as an application that replicates itself. This self-replication is a key characteristic that defines viruses. When a virus infects a host system, it can create copies of itself, often without the user's consent, and can spread to other systems through various means, such as email attachments, file transfers, or network connections. The intent of a virus often includes damaging data, stealing personal information, or rendering part of a system unusable.

In contrast to this definition, the other choices refer to different types of software with distinct functions. Specifically, options related to malware protection, encryption, and system backups serve protective and utility roles in computing, but none involve the self-replicating aspect that characterizes a virus. Thus, the correct description encompasses the essence of a virus in the digital landscape.