The best description of the type of plot structure used by the author in this excerpt from "Dusk" by H.H. Munro (Saki) is linear.
Why?
In this passage, the narrative unfolds in a straightforward chronological manner, presenting the setting and the character's reflections without jumping back to previous events (flashback) or presenting multiple storylines that might run alongside each other (parallel). There is no indication of future events that might be hinted at (foreshadowed) either. The focus is on the current scene and the protagonist's thoughts in the moment, which aligns with a linear plot structure.