The conditions most favorable to wave formation over mountainous areas are a layer of:

The conditions most favorable to wave formation over mountainous areas involve a layer of stable air at mountaintop altitude combined with a wind of at least 20 knots blowing across the ridge. In this scenario, stable air is less likely to rise and mix vertically, which allows for the smooth, horizontal oscillations that characterize wave formation. When a strong enough wind flows over the mountains, it creates a disturbance, leading to the development of gravity waves in the atmosphere.

The wind speed is critical; a minimum of 20 knots helps to create sufficient energy and lift to initiate and sustain these waves. The stable air contributes to the formation and persistence of wave patterns, resulting in features like mountain waves or rotor clouds downstream of the ridge.

In contrast, the other options describe conditions that either do not promote wave formation or are less favorable. Unstable air, for example, encourages vertical mixing and turbulence, which can disrupt wave formation. Lower wind speeds or dry air with high temperatures also do not provide the necessary dynamics for wave development over mountain regions.