She is wearing . . . . . . . . unusual dress.

The correct answer is D. no article.

The word "dress" is a common noun, which means it can refer to any type of dress. In this case, the speaker is not referring to a specific dress, but rather to any dress that is unusual. Therefore, we do not need to use an article.

If the speaker were referring to a specific dress, we would use an article. For example, if the speaker said, "She is wearing the dress that I bought her for her birthday," we would use the definite article "the."

We would also use an article if the speaker were referring to a type of dress. For example, if the speaker said, "She is wearing a long dress," we would use the indefinite article "a."

However, in this case, the speaker is not referring to a specific dress or a type of dress, but rather to any dress that is unusual. Therefore, we do not need to use an article.