Final answer:
The correct answer is C, which is effective positive feedback that acknowledges the student's effort and perseverance in solving a tough problem.
Step-by-step explanation:
Among the teacher statements given, the one that illustrates an example of appropriate and effective positive feedback is "C. This was a tough problem, but you worked hard until you solved it." This statement encourages perseverance and recognizes the student's effort, which is crucial in fostering a growth mindset. The feedback is specific and clear, pinpointing the difficulty of the task and the student's success in overcoming it. Such feedback is beneficial as it acknowledges the student's hard work rather than innate ability or competition among peers. In contrast, the other options either set up unnecessary competition, potentially set unrealistic expectations, or diminish previous success.