“Mark Twain” is Samuel Clemens’ pen name. If most people haven’t read anything by Samuel Clemens, it means most people haven’t read anything by Mark Twain. Therefore, this answer doesn’t explain why more people know that Mark Twain is a writer.

This provides a similarity between people’s awareness of Mark Twain and Samuel Clemens, but we need a difference. (B) tells us that of all the people who know Samuel Clemens was a writer, those same people also know Mark Twain was a writer. This doesn’t explain why so many more people know about Twain than Clemens.

This points out a difference in people’s knowledge about Mark Twain and Samuel Clemens. If most people don’t know that Mark Twain is Samuel Clemens, it provides a possible explanation for why far more people know that Mark Twain was a writer than Samuel Clemens.

Even if people believe conflicting things about Mark Twain and Samuel Clemens, it doesn’t address why far more people are aware that Mark Twain is a writer.

This explains why some people could be equally aware of both Mark Twain and Samuel Clemens, but we need an answer choice that does the opposite. We need a difference between people’s awareness of Twain and Clemens.