Metacognition's best definition can be described as an awareness and understanding of one's own thought processes. Answer A is obviously the incorrect choice as it refers to English, not psychology. Answer choice B makes sense as those who understand their own thought processes may be good test takers. Answer choice C is about learning new things which I would say does not necessarily apply to understanding what you know. It is about new knowledge whereas metacognition is about old knowledge. The final answer choice, D, does not apply since metacognition is not a process, but an awareness. I would say that the most applicable answer choice is B.
Hope that helps!
May your pencils stay sharp.