Concept:Good treatment of neighbours and guests is the subject of the 15th hadith in an-Nawawi's collection.Explanation:The 15th hadith of an-Nawawi teaches that whoever believes in Allah and the Last Day should speak good or remain silent.It also instructs believers to be generous and kind to their neighbour.It further instructs believers to honour and treat their guest well.Therefore, this hadith directly discusses good treatment of neighbours and guests.Answer:B. 15th of an-Nawawi