Explanation:

Different types of computer viruses target different parts of a computer system or software environment. **Analysis of Options:** * **Option (A) Macro virus:** Correct. Macro viruses are specifically designed to infect documents and templates created by application software that supports macros (e.g., Microsoft Word, Excel, PowerPoint). They are written in macro languages (like VBA) and execute when an infected document is opened, spreading to other documents or templates within the application's environment. This directly corresponds to spreading in application software. * **Option (B) Boot virus:** This type of virus infects the boot sector of a storage device (hard drive, floppy disk). It activates when the computer starts up, before the operating system fully loads, and does not primarily spread through application software or documents. * **Option (C) File virus:** File viruses infect executable files (e.g., .exe, .com, .dll). While these files are part of or used by application software, macro viruses are a more specific and direct answer for viruses that spread *within* the application software's document ecosystem by exploiting its features. * **Option (D) Antivirus:** An antivirus is a type of software designed to detect, prevent, and remove malicious software, including viruses. It is a protective measure, not a type of virus itself.