Concept:Umar's most notable achievement was not the capture of any town but the establishment of a widely known Islamic brotherhood.
Explanation:Tijaniyah is a Sufi Islamic order that began in the Maghreb and later spread into the northern parts of the Sudan region.
In West Africa, Umar, commonly known as Al-Hajj Umar Tall, became an important promoter of this order.
Through his teaching and jihad campaigns, he helped to establish and spread Tijaniyah among the people.
This made Tijaniyah a significant religious movement in the area.
Thus, among the listed options, it is the religious order, not Timbuktu, Bambara, or Djallon, that matches Umar's contribution.
Answer:C. Tijaniyah