Concept:The Day of Arafat is the most important day of Hajj, observed on a specific date in Dhul-Hijjah.Explanation:The pilgrims leave Mina and gather at Arafat on the ninth day of Dhul-Hijjah.This day is known as Yaumul Arafah.It is the core ritual of Hajj, and the Prophet (SAW) said Hajj is Arafat.Among the options given, the correct day is the ninth.Answer:C. ninth