Final answer:
Schist forms at a medium grade of regional metamorphism, a process where rocks are altered by high temperatures and pressures.
Step-by-step explanation:
The form that arises at a medium grade of regional metamorphism is schist. During the process of regional metamorphism, rocks are subject to both high temperatures and pressures, leading to physical and mineralogical changes. Initially, the parent rock, known as the protolith, can transform into slate at lower grades. As the grade progresses, the slate can become phyllite, which is a low to intermediate grade metamorphic rock. Further metamorphism causes the phyllite to transition into schist, characterized by more pronounced foliation and the growth of larger mica crystals. High-grade metamorphism would typically result in the formation of gneiss, which is not the correct answer in this case as we are specifically seeking a medium-grade metamorphic rock. Thus, 'b. schist' is the correct answer to your question.