You really should not be penalising for the extra article "a", how are people who are not absolutely fluent in English supposed to know this? Also, why not "this is" as opposed to "it is"?
This is and it is are the same in this case. As for your complaint about the article, this is a course meant for native speakers, so it doesn't make much sense for them to accept a wrong answer. I understand the frustration, but that's just how it works!