Option (A) is the correct answer. The provided text explicitly classifies the MTT assay as a 'Metabolic viability assay'. The text explains that the MTT salt reacts with mitochondrial dehydrogenase enzymes found in live cells, reducing it to formazan crystals. Since the amount of formazan produced is proportional to the cell number and relies on enzymes present in living cells, the assay is used to determine cell viability.