I think it would be answer D, to introduce an idea because it's introducing the idea of a Venus Fly Trap's diet.
Answer:
B
Explanation:
In MLA style the authors detail is written in parenthesis and the page number where it can be found.
Another way to put it could have been
Horner said:" this was an army trained on Horseback/ or, where the ground required, on foot".(55-56)
