Which sentence does not use a comma properly?
a. if you are interested in rare books, you must travel to the thomas fisher library in toronto.
b. the oldest, scholarly library is located in finland.
c. to prepare for the unveiling of the new exhibition, the docent requested the presence of president clinton.
d. the library tour guide, an elderly man of average height, is well versed in ancient literature.

Your answer is B!

A is used correctly because it's a continuation of the first thought, which is introducing you to the topic of the sentence. (the Thomas Fisher library in Toronto is the topic.)

C is used correctly because it provides a pause properly.

D is used correctly because it adds some separation (for clarity) in the two facts about the topic of the sentence. (The library tour guide is the topic.)

For B, it is used incorrectly because there's no need for that comma there. "The oldest scholarly library..." is fine without one.