Respuesta :
Answer:
D) When the artist painted the masterpiece
Explanation:
Subordinate clauses can act as an adjective, a noun or an adverb in a sentence.
When the artist painted the masterpiece is a noun clause acting as the subject of the sentence. It is answering the question "What is a real mystery?"
When the artist painted the masterpiece is correct. The subordinate clause is the subject of the sentence.