Explanation:
In the Mahabharata, Bhishma received the boon of choosing the time of his death from his father, King Shantanu. The story goes that Shantanu fell in love with Ganga, who later became his wife. However, Ganga had a condition that Shantanu should never question or interfere in her actions. Ganga, in turn, had to raise their sons, but due to various circumstances, she ended their lives. When Ganga was about to drown their eighth son, Devavrata (who later became Bhishma), Shantanu could not bear to lose another child. He questioned Ganga's actions, breaking his promise. Ganga then revealed her divine identity and took Devavrata away, promising Shantanu that he would not have to suffer the pain of losing another son. As a result of this incident, Devavrata, out of his love for his father, made a vow of lifelong celibacy (Bhishma Pratigna) and unconditional service to the throne of Hastinapura. In return for this sacrifice, Shantanu granted him the boon to choose the time of his death, giving Bhishma control over when he would leave his mortal coil. This boon allowed Bhishma to remain alive until the end of the great war, the Kurukshetra War, where he played a crucial role.