Hard choosing from "A" to "D", but my best answer that I came up with is "D" or "4"
The word "Who's" is a plural and actual stands for "Who is" So if you put that in that sentence "Who is book about blue whales is this?" Or for "2" the word "Whomever's" doesn't make since in the sentence.. "Whomever's book about blue whales is this?" And the same with "3". But with "4" "Whose" It fits in that sentence.. "Whose book about blue whales is this?" It's best if you read the sentence out loud and see which word best fits in that sentence.
