Answer: D) Itself.
Explanation: Reflexive pronouns are words ending in -self or -selves that are used when the subject and the object of a sentence are the same. In the given sentence, the subject and the object are the same (each of the horses). From the given options, the pronoun that correctly completes the sentence, is the corresponding to option D: itself, so the sentence would be: Each of the horses ate a flake of hay by itself.