When water waves approach the shore and slow down, the wavelength decreases. This is because the wave speed is related to frequency and wavelength by the equation:
\[ \text{Wave speed} = \text{Frequency} \times \text{Wavelength} \]
As the wave slows down (decreases in speed), and since the frequency remains constant (as it is determined by the energy source generating the waves), the wavelength must decrease in order to maintain the relationship.
So, the correct response is:
The wavelength decreases.