The correct answer would be: A. La
In the previous sentence from Juan he is asking Eric if he knew of the history teacher. Maestra is feminine since it has the -a at the end and la at the beginning indicating that the history teacher is female.
This is why la is the answer since the teacher is feminine. It wouldn’t be B since that’s masculine and it wouldn’t be C or D since those ,San more than one and there is only one history teacher.
Hope this helps! :3