Concept:The first treaty of Aqabah was an early pledge of allegiance made by a group from Yathrib to the Prophet Muhammad (SAW).Explanation:The Yathribites who accepted Islam at the first treaty of Aqabah were from the tribes of Aws and Khazraj.Among them was Al-Bara' b. Ma'rur, a leading man of the Khazraj tribe.He was one of the twelve representatives who took the pledge with the Prophet (SAW).The other options are not correct because Abbas b. Abdul Muttalib and Mus'ab b. Umayr were not Yathribites at that event.Abu Qays b. al-Aslat accepted Islam later and was not among the first Aqabah group.Answer:Option A: Al-Bara' b. Ma'rur.