Final answer:
The rogue software programs that generate spam are called 'bots,' which is a term used for automated software applications that perform repetitive tasks over the Internet.
Step-by-step explanation:
The four-letter word used to describe "rogue... software programs" that generate millions of spam messages is bots. Bots, or Internet robots, are automated software applications that run tasks over the Internet. These tasks can range from indexing web pages for search engines to launching spam or malware attacks. Option a) Bots is the correct answer.