I was wondering if someone please help me with this question ? It does seem to make much sense to me.

Open shelves are recommended for high-volume file rooms. Which of the following is a possible disadvantage of open-shelf files ?

A increased operational effciency
B less expensive than file
C 36 inches necessary for aisles between units
D faster pullability of records

My text book does say and I am not sure if there is a web-site that can help

I was thinking it might be C

Since your question asks about disadvantages, C must be the answer because the others appear to be advantages.