I just realized I didn’t answer your whole question.
This problem, “phantom answers,” comes up because, when a new word is entered into the database, multiple choice answers are automatically chosen from existing database entries and are attached to the new word. Those multiple choice answers remain with the word permanently, i.e., a multiple choice answer isn’t removed from the answer set, even when it is “removed” from the database. As a result, problems like you encountered can arise.
Memrise knows about the issue, but they haven’t addressed it.
Several users have devised solutions:
a Python solution
a userscript solution
[Userscript] Attempt to Eliminate Phantom Entries
I use the “one at a time” version of the Python solution. When a word turns out to have confusing multiple choice solutions attached to it, I go to the word in the database and add a character to each field for that word and save the modified entry. That bumps out the old multiple choice solutions and new multiple choices are made from the current database. Then I go back and remove the extra character from each field, so the entry looks correct again. It works well for occasional troublesome multiple choice issues (n.b.: This solution doesn’t work for phantom sound files).
You may not want that much information, but if you are ever creating a course yourself, this little tidbit could be very useful to you.