1) The only way it is possible for the smallest value in a set to be equal to the set's mean is if all of the numbers in that set are the same. This is not enough information to determine how many books have more than 250.

INSUFFICIENT

2) Telling us that the median is 240 does not tell us enough to determine how many books have over 250 pages.

INSUFFICIENT

1+2) One tells us that all of the books have the same amount of pages in them, while two tells us that the median is 240. As we know that one book has 240 pages, we know that all of the books have 240 pages and therefore not a single book has over 250 pages.

SUFFICIENT

Answer C