Concept:The Port-Harcourt railway line was built to transport a heavy mineral that could not be moved efficiently by road.Explanation:Coal is a very heavy mineral. Road transport was not suitable or economical for moving such heavy freight over long distances. The discovery of large coal deposits at Udi (near Enugu) created the need for a reliable railway line from Port-Harcourt into the hinterland to evacuate the coal. The railway allowed the coal to be moved quickly and in bulk to the port for export.Answer:Discovery of coal at Udi. (C)