The correct answer is C) 1.3% to 6.0% by volume. The explosive range, or flammable range, of diesel oil mixed with air is 1.3% to 6.0% by volume. This means that if the concentration of diesel oil vapor in the air is between 1.3% and 6.0% by volume, the mixture can be ignited and cause an explosion. This information is based on the standard explosive range data for diesel fuel, as specified in maritime safety regulations and guidelines for the handling of flammable substances. The other options are incorrect because 0% to 1% (A) is too narrow, 0.8% to 5.3% (B) is slightly off the mark, and 6.3% to 12.1% (D) is too wide for the explosive range of diesel oil.