27.
Your Answer is
Correct Answer is C
Explanation
The subject of the sentence is Ballet Nacional de Cuba is recognized ... Therefore, the horizontal part cannot have a new subject-verb, and A/B excludes. According to the meaning of the sentence, it means "already used as a training place for world-class dancers", so having done [completed] has the correct form.
Item D: served as... is a non-predicate done, meaning passive, which means wrong.