Study Review mode in exams offers an unlimited amount of time for you to answer the questions. They also show the correct answer to the question once you have answered, and allow you to see the question explanation during the exam.
Test Mode (Timed) exams create a test environment where you have a limited amount of time to answer a question. You can customize the amount of time per question to your specifications. The answer to the question and question explanation is not visible in this mode.