I choose the correct answer which is D, But what is wrong with B?

Hi Tyler808, (B) is wrong because it makes an assumption about most birds that is unwarranted from the stimulus. We know that most birds that are killed by predators have small spleens, but this tells us nothing about how many birds there are with small spleens overall. Lets look at an example. Let's say that 100 birds are killed by predators, and most of them (let's say 60), were small-spleen birds. In this case most of the birds killed had small spleens. Does this mean that most birds with small spleens are killed by predators? That depends on how many birds have small spleens overall. If only 115 birds have small spleens, then yes, but maybe there are 200 small spleen birds, or 2000, or 20000, or 2 million. We have no clue how many total birds there are with small spleens, so we cannot possibly say that most of them are killed.