When water waves approach the shore and slow down, the wavelength decreases. This is because the speed of the wave is related to its frequency and wavelength by the equation:
\[ \text{Speed} = \text{Frequency} \times \text{Wavelength} \]
As the speed decreases (due to the waves entering shallower water), and since the frequency remains constant (the number of waves passing a point per second does not change), the wavelength must decrease to maintain the equation.
So, the correct statement is:
The wavelength decreases.