The correct answer is B) 1% to 5% by volume. Bunker C, also known as heavy fuel oil, has a relatively low flammability range when mixed with air. The explosive range, or the range of concentrations in which Bunker C can ignite and explode, is between 1% and 5% by volume of Bunker C in air. This is the standard explosive range for this type of heavy fuel oil, as specified in the regulations and guidelines for the handling and storage of Bunker C. The other answer choices are incorrect because the explosive range is not 0% to 1% (too narrow), 5% to 10% (too high), or 10% to 20% (too high) by volume of Bunker C in air.