Concept:The Day of Arafah is the most important day of Hajj, and it occurs on a fixed date in Dhul-Hijjah.Explanation:Pilgrims leave Mina and gather at the plain of Arafah on the 9th day of Dhul-Hijjah.Standing at Arafah is the core ritual of Hajj, known as Wuquf.The Prophet Muhammad (SAW) said Hajj is Arafah, meaning this day is essential.Therefore, the correct option is the ninth day.Answer:A. Ninth