The given question relates to computer viruses, specifically identifying the type of virus that attaches to executable files and activates upon file execution.

Let's look at each option provided:

1. Macro Virus: This type of virus typically infects documents such as Word or Excel files by embedding itself in macros. Macros are small programs within a document that automate tasks. Macro viruses activate when the document is opened, not when an executable file is run.
2. Boot Sector Virus: These viruses infect the boot sector of a storage device like a hard disk or a floppy disk. They execute when the computer is started, making them active during the initial boot process, not specifically when an executable file is run.
3. File Infector Virus: This class of virus attaches itself to executable files such as .exe or .com files. When such a file is run, the virus activates and can spread further by infecting other executable files. Thus, this fits the description given in the question.
4. Worm: Worms are self-replicating programs that spread across network connections without needing to attach themselves to other files or programs. They do not require attaching to executable files specifically to activate.

Based on the detailed analysis above, the correct answer is File Infector Virus as they attach to executable files and activate when these files are executed.