8 files declare their use of QuizQuestion in Quiz 8.4

ClozeQuestion.php in question_types/cloze/lib/Drupal/cloze/ClozeQuestion.php

The main classes for the multichoice question type.

DDLinesQuestion.php in question_types/quiz_ddlines/lib/Drupal/quiz_ddlines/DDLinesQuestion.php

The main classes for the quiz_ddlines question type.

LongAnswerQuestion.php in question_types/long_answer/lib/Drupal/long_answer/LongAnswerQuestion.php

Long answer classes.

MatchingQuestion.php in question_types/matching/lib/Drupal/matching/MatchingQuestion.php

The main classes for the matching question type.

MultichoiceQuestion.php in question_types/multichoice/lib/Drupal/multichoice/MultichoiceQuestion.php

The main classes for the multichoice question type.

ScaleQuestion.php in question_types/scale/lib/Drupal/scale/ScaleQuestion.php

The main classes for the short answer question type.

ShortAnswerQuestion.php in question_types/short_answer/lib/Drupal/short_answer/ShortAnswerQuestion.php

The main classes for the short answer question type.

TrueFalseQuestion.php in question_types/truefalse/lib/Drupal/truefalse/TrueFalseQuestion.php

Defines the classes necessary for a True/False quiz.