Concept:Limestone changes into marble when it undergoes metamorphism.Explanation:Limestone is a sedimentary rock made of calcium carbonate.Great heat and pressure cause it to recrystallise and form marble.This change is called metamorphism.Slate comes from shale or mudstone, not limestone.Graphite forms from organic carbon, and gneiss forms from granite.So only marble is the correct product of limestone under these conditions.Answer:C. marble