The correct answer is: A. few
The word “few” is used to refer to a small number of people or things. In this case, the number of students who could get through the exam was small, so the word “few” is the most appropriate choice.
The word “a few” is also used to refer to a small number of people or things, but it is slightly more informal than “few.” In this case, the exam was likely very difficult, so it would be more appropriate to use the more formal word “few.”
The word “little” is used to refer to a small amount of something. In this case, the number of students who could get through the exam was small, but it is not necessarily the case that they were not able to get through the exam because they did not know enough. It is possible that they were not able to get through the exam because the exam was very difficult. Therefore, the word “little” is not the most appropriate choice.
The word “little” can also be used to refer to a small amount of time. In this case, the exam was likely very long, so it would be more appropriate to use the word “few” to refer to the number of students who could get through the exam.