The correct answer is B) an explosion may occur if a source of ignition is present. When diesel fuel vapors in a compartment are within the flammable range, it means the concentration of the vapor-air mixture is between the lower and upper explosive limits, making it capable of igniting and causing an explosion if a source of ignition is present. This is the critical condition that needs to be avoided. The other options are incorrect because: A) the mixture being too rich to burn is not the issue, C) the upper explosive limit being exceeded means the mixture is too rich to ignite, and D) the mixture being too lean to burn is also not the problem in this scenario.