The best description of the type of plot structure used by the author in this story is linear.
A linear plot structure presents events in a chronological order, following a straightforward progression from the beginning to the end without significant shifts in time or perspective. In this excerpt, the narrative unfolds in a direct manner, depicting Norman Gortsby's current thoughts and observations without jumping back (flashback) or forward in time (foreshadowing). The description of the park, the evening, and Gortsby's reflections happen sequentially, maintaining a logical flow that aligns with the linear structure.
While “parallel” might imply a simultaneous narrative involving different characters or timelines, there is no evidence of that in this passage. Thus, the overall structure best fits a linear plot.