To determine which type of computer virus attaches itself to executable files and gets activated when the file is run, we first need to understand the characteristics of different types of computer viruses.

1. File Infector Virus: A file infector virus is a type of virus that attaches itself to executable files, such as .exe or .com files. When the infected file is executed, the virus is also executed, allowing it to spread to other files and potentially cause damage or replication.
2. Worm: Worms are self-replicating malware that do not need to attach themselves to a host file, unlike file infector viruses. They exploit vulnerabilities in network security to spread across computers.
3. Boot Sector Virus: Boot sector viruses are designed to infect the master boot record of storage devices. They get activated when the system is booted from the infected device.
4. Macro Virus: Macro viruses infect documents that use macro languages, often found in software like Microsoft Word or Excel. They are executed when an infected document is opened.

Given these definitions, the correct answer is File Infector Virus because it specifically attaches itself to executable files and becomes active when the file is run.

Let's rule out the other options:

• Worms do not attach to files but replicate independently.
• Boot Sector Viruses target boot records, not executable files.
• Macro Viruses target macros within documents, not standalone executable files.

In conclusion, based on the characteristics and mode of operation, the File Infector Virus is the correct answer as it matches the description provided in the question.