The application will ask questions like:

What comes after D?

A child is expected to answer the question and the application will inform the child if the answer is correct or wrong. The application will ask a total of 20 questions. After all 20 questions have been answered, the application will display a score (from 0 to 20) to inform the child how many questions he or she answered correctly. The application will also list all the letters that were asked.

The letters tested are from ‘A’ up to ‘Y’. Do not include the letter ‘Z’. The letters are selected randomly. No letters are to be repeated, that is, each letter is only asked once in the 20 questions.

Note: The java.util.Random class can be used to generate random numbers. For example, the following statements may be used to generate a random number from 0 to 4 inclusive:

Random generator = new Random();
int number = generator.nextInt(5);

can anybody help me on this question?

Your teacher can. He's of course the one you should have listened to when he tried to teach you Java.
Or are you just too lazy to do your own homework?