Concept:A diagnostic test identifies a learner's specific strengths and weaknesses in a subject or skill.Explanation:Diagnostic tests are qualitative in nature. They help find gaps in understanding. They show where additional teaching is needed. They also point out areas where a student excels. The goal is to plan remedial instruction based on individual needs.Thus, a diagnostic test does not rank learners or test summarising ability. It focuses on pinpointing both strong and weak areas in language.Answer:Option D – learners' strong and weak points in language.