When flying at high speeds and low altitudes, a helicopter faces several risks, but the most immediate and concerning risk is the reduced reaction time. At higher speeds, the time available for the pilot to respond to changing conditions or emergencies is significantly diminished. The proximity to the ground also limits the pilot's ability to execute necessary maneuvers to avoid obstacles or manage upsets, increasing the chances of an accident.

• Reduced reaction time due to speed and altitude: This is correct because the faster the helicopter travels, the less time the pilot has to perceive and react to any changes or threats in the environment. In the event of the engine failure, there is insufficient height for the pilot to identify the failure, make the appropriate control inputs and for the pitch attitude to change sufficiently before the helicopter impacts the ground.
• Over torque of the engine or transmission: While over-torque can indeed be a risk under certain high power demand situations, it is less specific to the combination of high-speed and low-altitude flight compared to reaction time issues.
• Retreating blade stall from decreased airflow: Retreating blade stall is a condition that can indeed occur at high speeds, particularly when a helicopter exceeds its Velocity Never Exceed (VNE) limit. In this scenario, the airflow over the rotor blades changes, leading to a situation where the retreating blade (the blade moving away from the direction of flight) experiences a reduction in lift as its angle of attack increases beyond the stall angle. This stall can result in a loss of control, leading to potential accidents. However the question is not specifying
• Mast bumping, especially in older tethering helicopter models: Mast bumping is typically associated with low-G pushovers in helicopters equipped with teetering rotor heads, where the blades can experience significant movement relative to the mast due to sudden changes in aerodynamic loading. In the context of the question, which does not specify the type of rotor system, mast bumping can be considered less relevant.

Note: All answers can be correct however, in a high-speed, low-height scenario, reduced reaction time poses a significant risk to helicopter safety, making it the most accurate statement among the options provided.