D. 5ft
Explanation:
The wave base of a wave is the depth at which there is no more perturbation of the medium caused by the wave. The wave base of a wave is determined to be half the wavelength of the wave itself.
In this problem, we have a wave with wavelength of 10 ft, so the wave base is
[tex]b=\frac{\lambda}{2}=\frac{10 ft}{2}=5 ft[/tex]
