What combination of factors is likely to reduce helicopter performance the most?

The combination of high altitude, high temperature, and high humidity significantly impacts helicopter performance, making this option the most challenging scenario for helicopter operations.

High altitude reduces air density, which in turn decreases the efficiency of the rotor blades and engines. Helicopters rely on a specific amount of lift generated by the rotor blades, and reduced air density at higher altitudes means that they must work harder to generate the same amount of lift, potentially leading to performance limitations.

High temperature further exacerbates this issue. Hot air is less dense than cold air, which diminishes the already reduced lift capabilities further, making it difficult for the helicopter to operate efficiently.

Additionally, high humidity can also contribute to poorer performance. Water vapor is less dense than air, so more humid air further decreases the effective air density, compounding the difficulties already presented by altitude and temperature.

Thus, this combination of factors creates a situation where the helicopter is significantly less capable of maintaining lift and performance, which is why this option represents the most severe reduction in helicopter performance.