2017-09-20 I realize it's an obsolete sense of "pen" in English, but why isn't plume accepted?
Even so, it's ironic that "stylo," very likely comes from Latin stylus, an equally obsolete pointed stick that young ancient Romans used for practicing their letters on wax-coated tablets. Writing in wax could easily be erased for the next lesson.
I think you answered your own question there: it's obsolete in English, so the course creators didn't think to add it as an alternative. Make sure you report it the next time it comes up so they can add it as an alternative answer.
"Plume" can be accepted but we must understand that it is more specific to a "quill", i.e., a sharpened bird-feather, that is dipped into a bottle of ink. For what it's worth, WordReference does not even list "pen" as one of the definitions for "plume". So I think we would have to agree that your assessment is spot on: it is an obsolete term. http://www.wordreference.com/fren/plume It seems that the term "stylo" has captured the scene when it comes to pens. Even a fountain pen is now called a "stylo à encre".
Dont get why when I write "Ma stylo" it said must be "Ma plume" then wrote "Mon Plume" and says has to be "Mon Stylo"?