The sentence that is most clearly from a story with a retrospective narrator is option c: back before I got married, I was once a solider in the union army
The sentence "Back before I got married, I was once a soldier in the Union army" suggests a narrator who is reflecting on a past period in their life. The use of the phrase "back before" indicates a temporal distance from the current moment of narration.
So, The narrator is providing information about their past, specifically about being a soldier in the Union army before getting married. This retrospective perspective implies that the narrator is recounting events that occurred earlier in their life, giving the narrative a reflective and reminiscing quality.