C. Conducting a mini-lesson on how to write a topic sentence and a conclusion
In this scenario, where the teacher has identified that the ELL's writing is unorganized, conducting a mini-lesson on how to write a topic sentence and a conclusion is the most appropriate next step. This addresses the specific issue of organization in the student's writing and provides guidance on fundamental elements of structuring a narrative. It offers practical support for improving the student's writing skills and is directly related to the observed problem. Options A, B, and D do not directly address the issue of organization in the writing as effectively as option C does.