Final answer:
The principal restoring force for wind-driven waves in the sea is gravity, which acts to return the water to its equilibrium level when it is displaced by wind. Option number correct.
Step-by-step explanation:
The principal restoring force for wind-driven waves in the sea is gravity. When a wave forms, the water in the wave rises above the equilibrium level, and gravity acts to restore the water to that level. Similarly, when the water dips below the equilibrium level, gravity pulls it back up. While other forces such as buoyancy, surface tension, and Coriolis force can influence the behavior of waves, they do not primarily act to restore the wave's shape.
Buoyancy primarily influences whether objects float or sink in the fluid, and surface tension can affect the formation of smaller ripples on the water's surface. The Coriolis force affects the direction of wave propagation but is not a restoring force.