Concept:Rocks are grouped by how they form. Quartzite, schist and marble all form through metamorphism.
Explanation:Marble forms when limestone is changed by heat and pressure.
Schist forms when shale or mudstone is transformed under intense temperature and pressure.
Quartzite forms when sandstone is altered by heat and pressure during metamorphism.
In each case, the original rock changes its structure and minerals without completely melting.
These changed rocks are grouped as metamorphic rocks.
They are not igneous rocks, because they did not cool from molten magma.
They are not sedimentary rocks, because they were not formed by the deposition of sediments.
Answer:A. Metamorphic