Final answer:
Mark Twain, whose actual name is Samuel Langhorne Clemens, used pen names such as Thomas Jefferson Snodgrass and W. Epaminondas Adrastus Blab before settling on Mark Twain.
Step-by-step explanation:
Before adopting his famous pen name, Mark Twain, Samuel Langhorne Clemens used several other pseudonyms. One of the pen names he chose before Mark Twain was Thomas Jefferson Snodgrass. Another was W. Epaminondas Adrastus Blab. It's worth noting that Samuel Clemens is not a pen name but his actual given name. Therefore, the correct answer to the question of what pen names Mark Twain chose before the name Mark Twain is option (a): Samuel Clemens, Thomas Jefferson Snodgrass, W. Epaminondas Adrastus Blab.