The correct answer is: A few.
The word “a few” is used to refer to a small number of things or people. In this case, the question is asking how many works of reference are as valuable as Encyclopedia Britannica. The answer is “a few,” because there are only a small number of works of reference that are as comprehensive and authoritative as Encyclopedia Britannica.
The other options are incorrect because they do not accurately reflect the number of works of reference that are as valuable as Encyclopedia Britannica. The word “few” is used to refer to a small number of things or people, but it does not necessarily mean that there are only a few of them. The word “the few” is used to refer to a specific group of things or people, but it does not necessarily mean that there are only a few of them. The word “fewer” is used to refer to a smaller number of things or people than something else. In this case, the question is not asking how many works of reference are less valuable than Encyclopedia Britannica, so the word “fewer” is not the correct answer.