Which Agatha Christie novel introduced detective Hercule Poirot?

Agatha Christie introduced Hercule Poirot in The Mysterious Affair at Styles.

Christie wrote the mystery during the First World War, in 1916, while she was working as a nurse and later in a hospital dispensary. The novel was first published in the United States in October 1920 and appeared in the United Kingdom on 21 January 1921. It introduced not only Poirot but also Captain Arthur Hastings, who narrates much of the story, and Inspector James Japp.

The plot centres on the murder of Emily Inglethorp at Styles Court, an English country house. Poirot, a Belgian refugee living in England, investigates the apparently baffling crime by examining alibis, motives, physical evidence, and the relationships among the household’s residents. His reliance on method, psychology, and what he calls his “little grey cells” became a defining feature of the later Poirot novels.

The other options are later Christie mysteries: The ABC Murders, Death on the Nile, and Murder on the Orient Express. Styles is therefore not merely an early Poirot adventure; it is the detective’s literary debut and the beginning of one of crime fiction’s most famous series.