Who obtained boon to kill bheeshma

Amba obtained the boon to kill Bhishma after being humiliated when Bhishma refused to marry her (he had abducted her along with her sisters for his brother Vichitravirya). After being rejected by Vichitravirya and even by her own lover, she performed severe penance and received a boon from Shiva to be the cause of Bhishma's death. She was then reborn as Shikhandi (to Drupada) and later became instrumental in Bhishma's death on the 10th day of the Kurukshetra war. Note that option C 'shikandhin' is her reincarnation, not the one who obtained the original boon.