Final answer:
In the storage of oxygen and fuel gas cylinders, they must be separated by a 20 ft. high wall.
Step-by-step explanation:
In the storage of oxygen and fuel gas cylinders, it is important to separate them for safety reasons. Therefore, the correct option is (c) - the cylinders must be separated by a 20 ft. high wall. This is necessary to prevent any accidental mixing of oxygen and fuel gas, which could lead to a highly flammable and dangerous situation.