...... works of reference are valuable as Encyclopedia, Brittanica.

Answer: Option A

A few works of reference are valuable as Encyclopedia, Britannica.
In this context, "a few" is used to indicate a small number of something. It suggests that while there may not be many works of reference that are valuable, there are still some, such as the Encyclopedia Britannica. Therefore, "a few" is the correct choice to complete the sentence, accurately conveying the intended meaning.