In "Dusk" by H.H. Munro (Saki), the event that is indicative of a circular plot is:
b. Gortsby and the elderly man return to the bench at the end.
This option suggests a return to the starting point, which is a key characteristic of a circular plot, where the narrative ends in a similar place to where it began, creating a sense of closure or repetition in the story's thematic elements.