To answer this question, just remember that you're looking to prove the story is in first person. Which excerpt provides a subject written in first person?
A. " red dust lay deep over everything” This describes the setting, so it's wrong
C. “through never-ending miles of ripe wheat” This also describes the setting
D. “the woodwork was hot to the touch” Once again, the subject is the setting
The only answer left is B, which states, “we sat in the observation car”. WE sat in the observation car. B is the only answer that includes a subject from a first person point of view, and therefore is correct.