This is . . . . . . . unique chance. A. a B. an C. the D. no article

The correct answer is: no article.

A unique chance is one that is not repeated or common. It is a chance that is special and important. Therefore, it does not need an article.

The other options are incorrect because they would imply that the chance is not unique.

• A unique chance would imply that there are other chances that are similar.
• An unique chance would imply that the chance is rare, but not unique.
• The unique chance would imply that there is only one chance of this kind.

In conclusion, the correct answer is: no article.