Final answer:
The correct rock that completes the sequence is schist.
Step-by-step explanation:
The correct rock that completes the sequence of increasingly metamorphosed rocks is schist.
The sequence of increasingly metamorphosed rocks is: slate → phyllite → schist → gneiss → migmatite.
Schist is a foliated metamorphic rock that forms as a result of high-grade metamorphism. It is characterized by the presence of visible mineral grains, such as mica and amphibole, and has a medium to coarse-grained texture.