Which of the following intensities of icing would be most hazardous for aircraft operations?

Severe icing is considered the most hazardous for aircraft operations due to its potential to significantly impact the performance and safety of an aircraft. When an aircraft encounters severe icing conditions, ice accumulates quickly and in large amounts on critical surfaces, such as wings and control surfaces. This mass and weight of ice can drastically alter the aircraft's aerodynamic characteristics, leading to reduced lift and increased drag.

Additionally, severe icing can lead to dangerous situations where the aircraft may experience complete loss of control if the ice is not removed promptly. Such icing could also affect instruments, systems, and the overall integrity of the airframe, compounding the risks involved in flight.

In contrast, trace, light, and moderate icing can still pose some hazards, but they do not typically result in immediate and dangerous changes to aircraft performance to the same extent as severe icing. Thus, when considering the safety of flight operations, severe icing represents the highest level of risk and is deemed the most hazardous.