What does a macro virus do?

A macro virus is a type of malware that targets applications, particularly those that handle documents, such as word processors and spreadsheets. It is written using the macro programming language within these applications, which allows it to automate certain tasks.

The correct answer highlights that a macro virus executes when a document is opened. This is significant because the virus is embedded within a document's macro code. When a user opens a document containing a macro virus, the malicious code executes automatically, often without the user's awareness. This can lead to damage or unauthorized actions such as corrupting files, stealing data, or spreading to other documents.

In comparison, the other options do not accurately reflect the behavior of a macro virus. Infecting an operating system during boot-up is more characteristic of a boot sector virus rather than a macro virus. The random infection of files is not a defining attribute of macro viruses either; instead, they specifically target documents with macros. Additionally, while some malware requires manual execution to spread, a macro virus can activate itself upon opening a document, making it distinct in its method of execution.