Correct answer is (a) Lactic acid
Explanation: Lactic acid comes under the first category of the class of metabolite that represents products directly associated with generation of energy in the cell. Compounds in the first category are formed directly as end- or by-products of energy metabolism; these materials are synthesised in pathways which produce ATP. Examples include: Ethanol, acetic acid, gluconic acid, acetone, butanol, lactic acid, other products of anaerobic fermentation.
