Online Quiz Application

Create an online quiz application that allows users to take quizzes, track their scores, and review answers. Quizzes can be in various formats (multiple choice, true/false, written). The system should handle different states, such as 'not started', 'in progress', and 'completed', and provide functionalities to save progress and submit answers. Discuss how to manage state transitions and user interactions gracefully, along with extensibility for new question formats.