The correct answer to this question is Abstract. The main part of this sentence that helps one know that it is abstract is the word-final, which indicates it has been completed. When it comes to classes, an abstract is a class that is incomplete.
This is why that keyword is unable to be used. For abstracts to work correctly, it is necessary for them to rely on other classes. Abstracts are not capable of surviving on their own. In the end, they work with other classes, and it is important to know those classes when programming language is being used.