The correct answer is A. To inform theatre critics about the merits of the Moscow Art Theatre.
The author holds a judgemental and harsh tone throughout the entirety of the article, specially towards american theatres administrators. Ostrovsky constantly remarks Moscow Art Theatre's points of superiority over american theatres as a means to praise the grandiosity of the former; he may believe that this theatre is not appreciated enough by critics whose articles incline towards more renown theaters - such as those of America publicized by their heavily popular media.
The rest of the options could be considered, but not important enough compared to the focus on the reputation of the Moscow Art Theater the author intends to aim at.
B: Staging productions of The Blue Bird and Hamlet are present on the article, but only briefly to evidence the author's main objective.
C: Naturalism in theatre is only present in the article for two paragraphs; not enough to detail it as a main concept.
D: The author mainly means to mock other theaters - specially americans - rather than to offer them improvement advice. This is evidenced by the harsh, mocking tone the author uses on his understatements everytime an America is mentioned.