The correct answer is D) Spontaneously combustible. Class 4.2 substances are classified as "Spontaneously Combustible" materials under the U.S. Department of Transportation's Hazardous Materials Regulations. These are solids that are liable to spontaneous heating or to heating up in contact with air, and being then liable to catch fire. The proper label name for this class of hazardous materials is "Spontaneously Combustible." The other answer choices do not accurately describe Class 4.2 substances. "Dangerous when wet" is the label for Class 4.3 materials, "Flammable solid" is the label for Class 4.1 materials, and "Flammable liquid" is the label for Class 3 materials, which have different hazard characteristics.