Books per dollar is books/dollars. Dollars per book is dollars/books.
Jo: 9/4=2 1/4 $/book
Kei: 12/8=3/2=1 1/2 $/book
Kate: 30/16=15/8=1 7/8 $/book
Bryn: 8/4=2 $/book
The lowest dollars/book means you can get more books for the same amount of money. So we need the lowest $/book, which is 1 1/2$/book or $1.50 per book. So we can buy more books at this cost. For example, 8 books costs Kei $12. But it would cost 8*2.25=$18 for 8 books at the rate Jo was paying, $15 for Kate, $16 for Bryn. Kei got the best deal, answer B.