Final answer:
Fully-developed wind-waves have reached their maximum growth, with energy transfer from the wind at its peak. White-caps are formed when the wave crests become unstable and break due to the continuous wind energy input.
Step-by-step explanation:
When we say that wind-waves are fully-developed (FD), it means that the waves have reached a state where they cannot grow any larger because the energy being transferred from the wind to the waves is at a maximum for the given wind speed and fetch (the distance over which the wind blows).
At this point, the waves contain large, mature swells and the sea is rough. White-caps are produced in a fully-developed sea as the tops of these waves become unstable and break, forming frothy, white crests that are visible from a distance.
This occurs because the wind energy continues to input into the waves, and when the gradient of the wave slope becomes too steep, it causes the crest to spill over.