Answer:
Option B is right.
Explanation:
Given that there are totally 9 books unread by Mrs. Reid. OUt of these she wants to take only 3 books on the trip.
For books order does not count.
Hence combinations would be appropriate to calculate.
3 books out of 9 can be selected in
9C 3 ways
No of ways of selecting 3 books for the trip out of 9 =

There are just 84 ways of choosing 3 books to bring on the trip out of 9 unread books.
Option B is right.