"Has the class already started?"
Why is this the right answer? Doesn't はじまっています mean "is starting" rather than "has started"?
ています is not just used for making "-ing" forms (start-ing), but also for showing a past transition of state that is still relevant in the present. (The class started earlier, and it is ongoing now.)
かれ·は·けっこんして·います。 He is married. (He became married, and so, is now married.)
かれ·は·はしって·います。 He is running. (This could also mean that "he runs", as a habit.)
Ah, others have said something like that, that class isn't just starting at this moment; it started, and is still in the "started" (ongoing) state. Something to get used to.
I answered the same as the solution, except that I put "already" (もう) in front of started, instead of in front of class. Why would "already" modify class instead of started? And in counting it incorrect, it said the error was at the end (under the す) - although I can see it getting confused.
[Of course, the teaching (web page only) also lists the various forms as including "profibit"!]
It's more like, "Already the class has started?" It's not interrogative, not even necessarily a question.