Final answer:
Migmatite (Answer A) best fits the description of a neosome predominantly composed of the solid fraction left after partial melting and extraction of the melt fraction. It is a mixture of both metamorphic and igneous components, often containing minerals like quartz and feldspar.
Step-by-step explanation:
The question relates to the process of partial melting in rock formation, particularly within the context of metamorphic and igneous processes. The term 'neosome' refers to the new material within a metamorphic rock that has undergone partial melting and recrystallization. The correct answer to the student's question is A. Migmatite. This rock is a mixture of igneous and metamorphic materials, which features both a crystalline structure from the melting (melted part) and a more solid structure (the residue).
Migmatites form when high temperatures during metamorphism result in partial melting. The melted fraction can segregate into lighter components, leaving behind a residuum which is the solid part remaining after the extraction of the melt. This residuum in migmatites will usually contain minerals resistant to melting such as quartz and feldspar.
Therefore, the student's description best fits the definition of migmatite, an example of a neosome predominantly composed of the solid fraction that remains after partial melting in a metamorphic rock context.