Mary often challenges herself to be aware of preconceptions and assumptions and to engage in reflective thinking. her friends sometimes accurately refer to her as a(n) thinker.

Critical

Explanation:

The critical thinking is the logical judgement that involve the conclusion and the argument. The critical thinking include the self-monitoring, self corrective thinking and the self-disciplined.

It also refers to the problem solving abilities and also refers effective communication standards as it engage in the reflective thinking. The critical thinking is the process of analyzing the various types of facts more effectively and briefly.

 According to the given question, Mary challenging herself for aware about the different types of perceptions and also assumptions and her friends accurately refer her as critical thinker.  

 Therefore, Critical is the correct answer.