Concept:The collection of Hadith was prompted by the loss of the companions who were the original oral preservers of the Prophet's sayings.
Explanation:During the lifetime of the Prophet Muhammad (S.A.W.), Hadith was preserved through the memory of his companions.
After his death, the companions continued to teach and transmit the Hadith to the younger generation.
However, with the passage of time, the number of companions who had personally heard the Prophet gradually decreased.
Each companion who died represented the loss of a direct living link to the sayings and actions of the Prophet.
This created a serious risk that authentic Hadith would be lost forever, so Muslim scholars began the task of collecting and writing down the Hadith.
Other factors like the growing tendency of forgery and the rise of Islamic centres of learning also supported the need for collection, but the main factor was the gradual decrease of the prophet's companions.
Answer:D. Gradual decrease of the prophet companion