The correct answer is: A. Few
The word “few” is used to describe a small number of things. In this case, the speaker is saying that there are only a few works of reference that are as valuable as the Encyclopedia Britannica.
The word “a few” is used to describe a small number of things, but it also implies that there are more than just a few. In this case, the speaker could be saying that there are a few works of reference that are as valuable as the Encyclopedia Britannica, but there are also many other works of reference that are valuable.
The word “the few” is used to describe a specific group of things. In this case, the speaker could be saying that there is a specific group of works of reference that are as valuable as the Encyclopedia Britannica.
The word “fewer” is used to describe a smaller number of things than something else. In this case, the speaker could be saying that there are fewer works of reference that are as valuable as the Encyclopedia Britannica than there are other works of reference.
In conclusion, the correct answer is “few” because the speaker is saying that there are only a small number of works of reference that are as valuable as the Encyclopedia Britannica.