Which wave property indicates how close the crests of waves are to each other?

The property that indicates how close the crests of waves are to each other is wavelength. Wavelength is defined as the distance between two consecutive crests (or troughs) of a wave. It essentially measures the length of one complete cycle of the wave. In graphical representations of waves, such as sine waves, the peaks (crests) are the high points, and the wavelength quantifies how far apart these peaks are.

Understanding wavelength is crucial for many applications in physics and technology, such as in the field of telecommunications or in understanding sound waves. While amplitude relates to the height of the wave and impacts the wave's energy, and frequency deals with how many cycles occur in a given time period, those properties do not provide information about the spacing between crests. Velocity describes the speed of the wave and is not associated with the distance between crests either. Thus, wavelength is the correct answer that corresponds to the distance between the crests of the waves.