Computer viruses are classified by what they attach to and how they trigger. The question describes malware that binds itself to executable program files and springs into action only when the host program is run — this is the defining behaviour of a File Infector Virus (also called a parasitic or program virus).

How a file infector works:

• It inserts its own code into executable files, typically those with .exe or .com extensions (and sometimes device drivers or overlay files).
• The virus normally alters the program's entry point so that when the user launches the file, the virus code executes first, then hands control back to the genuine program so nothing looks wrong.
• Once resident, it can locate and infect other executables, corrupt or delete data, and degrade system performance.

Why the correct choice fits and the others do not: A self-replicating program that spreads across networks without needing a host file is a worm — the key distinction is that a worm is standalone and does not attach to an executable, so it does not match the description. A virus that infects the macro code embedded in documents (such as Word or Excel files) is a macro virus; it activates when the document is opened, not when a program is executed. A virus that hides in the boot sector or master boot record loads when the computer starts up, before the operating system, rather than when a specific file is run. Only the file infector is triggered by executing an infected program, which is exactly what the question asks about.