To solve this question, the user needs to have knowledge of abstract classes and the hierarchy of classes used for writing characters.

The option A, FileWriter, is incorrect because it is a concrete class that extends CharWriter.

The option B, CharWriter, is incorrect because it is also a concrete class that extends Writer.

The option C, Writer, is the correct answer because it is the abstract class that serves as the superclass of all classes used for writing characters.

The option D, OutputStream, is incorrect because it is the superclass of all classes used for writing bytes.

The option E, FileOutputStream, is incorrect because it is a concrete class that extends OutputStream.

Therefore, the answer is: C. Writer