This is a detail question, which we should be able to back up the right answer choice with reference to the passage.
Benefits of herding is explained in 30-34.
A) Degree of "density" is not discussed. Out of scope.
B) Distinction between adults and young ones is not discussed. Out of scope.
C) It's rather opposite of what's stated in the passage. There is rather "increased competition for food." (34).
D) This is the paraphrase of "diminishes the chances of any particular animal" (33) (Correct)
E) While we can conjecture this, it's not directly referred from the passage.