Concept:When a student lacks fundamental concepts, the teacher's role is to identify the specific learning gaps and then provide targeted support to overcome them.Explanation:If a student is struggling with basic concepts, the first step is to figure out exactly *where* the problem lies. This is called diagnosis. A teacher needs to pinpoint the specific areas of difficulty or the reasons behind the student's errors. Once these learning difficulties are identified (diagnosed), the teacher must then provide special instruction or help to correct these weaknesses. This corrective teaching is known as remedial teaching. Therefore, the best approach is to both diagnose the problem and then implement remedial teaching.Answer:D. To diagnose and take up remedial teaching.