It is . . . . . . . . very popular dancing room. A. a B. an C. the D. no article

The correct answer is: no article.

The word “dancing” is a non-count noun, which means it cannot be used with a singular article (a/an). Non-count nouns refer to things that cannot be counted individually, such as water, sand, or music.

In this case, the speaker is referring to a general type of room, not a specific room. Therefore, the article “a” or “an” would not be appropriate.

The article “the” would also be incorrect, because it is used to refer to a specific person, place, or thing. In this case, the speaker is not referring to a specific dancing room, but rather to a general type of room.

Therefore, the correct answer is no article.