D. The word "ghosts" is used to represent Robert's war memories.
Explanation:
The passage begins as hopeful but melancholic but soon becomes a testament that Robert has returned from Vietnam with a case of PTSD. 'Ghost' is a poignant metaphor for his condition as it describes what past memories feel like to a person suffering from it.
The memories linger like a supernatural power, ever present, ever lingering but ever so out of touch and only seen by those effected by it. It is a great metaphor to show Robert's condition.