The exchange student in our department became __________ in the language after six months of practice.

The term that best fits the context of the sentence is "proficient." When someone becomes proficient in a language, it means they have achieved a high level of competence, ability, and skill in communicating effectively. The idea suggested in the sentence is that after six months of practice, the exchange student has developed enough fluency and understanding of the language to communicate well.

In contrast, the other terms do not align with the intended meaning. "Reflective" refers to the ability to think deeply or consider thoughtfully, which does not specifically relate to language skills. "Dormant" implies a state of inactivity or being temporarily unused, which would suggest a lack of progress rather than improvement in language abilities. "Redundant" signifies something that is no longer necessary or superfluous, which does not apply to the context of language proficiency.

Therefore, "proficient" is the correct choice as it accurately describes the positive outcome of the student’s intensive language practice over six months.