To determine the missing coefficient for water based on the law of conservation of matter, it's important to know the specific chemical equation you are working with. The law states that matter cannot be created or destroyed in a chemical reaction, so the number of atoms for each element must be the same on both sides of the equation.
If you provide the balanced equation that includes water, I can help you find the missing coefficient. Without that information, I can't provide the correct answer among the options given (6, 2, 8, 4). Please share the specific reaction or equation you're examining.