The correct answer is B) the fuel used is less volatile. Diesel engines are considered safer than gasoline engines because diesel fuel is less volatile and has a higher flash point compared to gasoline. This means diesel fuel is less likely to ignite or explode under normal operating conditions, making diesel engines inherently safer. In contrast, gasoline is more volatile and has a lower flash point, increasing the risk of fire or explosion in the event of a fuel leak or malfunction. The other options are incorrect because they do not directly address the key safety advantage of diesel engines. While diesel engines may operate at lower speeds (A) and be more heavily built (C), these factors do not make them inherently safer than gasoline engines. The ability to be easily reversed (D) is not a primary safety consideration when comparing diesel and gasoline engine technologies.