"This is an important play."
Translation:Αυτό είναι ένα σημαντικό θεατρικό έργο.
The multiple choice version of this question requires "Είναι ένα σημαντικό θεατρικό έργο" as an answer - even tho there is no "this" in that answer.
On another point, is the "θεατρικό" always necessary, or could it be left out in context - like in German where you would mostly say "Stück" rather than "Theaterstück", unless it was unclear what you were talking about?
Έργο also means work as in the effort that is uncountable. So I think it would be best to include the article when έργο refers to a countable work's product.
A general note: 1) while all correct alternatives should be included, sometimes one correct version is heavily favoured over another. 2) indefinite articles are not always dropped when they are optional. The problem is that it depends on the phrase and what sounds natural or clearer. And then of course, it's the verbal versus written Greek, where many things appear wrong on paper however normal they might sound! :)