Concept:The collection of Hadith became necessary due to the fear of losing authentic sayings of the Prophet (S.A.W).Explanation:During the early period, the companions of the Prophet (S.A.W.) preserved Hadith through memory and oral transmission.Over time, many companions began to pass away.Their gradual decrease created a serious risk that valuable Hadiths would be lost forever.This danger made it essential for scholars to gather, verify, and record the Hadith in written form.Answer:D. Gradual decrease of the prophet's companions