The correct answer is C. Objective.
A teacher must be objective in evaluating answer sheets in order to establish his credibility. This means that he must not let his personal feelings or biases influence his grading. He must also be fair and consistent in his grading, and he must provide clear and accurate feedback to his students.
A teacher who is strict may be seen as unfair or biased, and his students may not trust his grading. A teacher who is lenient may be seen as not taking his job seriously, and his students may not learn as much. A teacher who is prompt may be seen as efficient, but he may not have enough time to give his students accurate and helpful feedback.
Therefore, the best way for a teacher to establish his credibility in evaluating answer sheets is to be objective.