The defining characteristic of a mentor is someone who:

A mentor is defined by their genuine commitment to fostering the growth and development of their mentees. This relationship is characterized by the mentor's active involvement in guiding, supporting, and nurturing the skills and knowledge of the trainee. A mentor seeks to inspire and facilitate professional and personal growth through ongoing communication, feedback, and encouragement.

In contrast, the other options do not capture the essence of mentorship. For instance, having extensive knowledge while being unavailable to the trainee contradicts the fundamental aspect of mentorship, which relies on accessibility and a willingness to engage. Similarly, providing guidance solely during performance reviews offers limited support, as mentorship typically involves continuous interactions rather than sporadic assessments. Lastly, focusing solely on one's own research disregards the essential mentor-mentee dynamic, where the mentor prioritizes the mentee's needs and aspirations.

Overall, the defining characteristic of a mentor lies in their sincere interest in the development of their trainee, making option C the clear answer.