Final answer:
To say that one wave is out of phase with another means the waves are D) out of step, meaning their peaks and troughs (or compressions and rarefactions) do not coincide. This term refers to the alignment of the waves' cycles, not to differences in their frequency, amplitude, or wavelength.
Step-by-step explanation:
To say that one wave is out of phase with another is to say that the waves are D) out of step. Waves that are out of phase do not align with each other in their points of compression and rarefaction (in the case of longitudinal waves) or crests and troughs (in the case of transverse waves). As such, points that are not separated by a complete number of wavelengths are considered to be out of phase. For instance, points on two waves that are crests on one and troughs on the other, or that are half a wave cycle apart, would be out of phase. This does not necessarily relate to waves having different frequencies, amplitudes, or wavelengths, although different path lengths traveled can cause initially in-phase waves to become out of phase due to a phase shift.
The concept of phase is separate from frequency, which is the number of cycles per unit time; amplitude, which is the maximum displacement from an equilibrium position; and wavelength, which is the distance between successive crests or troughs. Two points along the wave that are separated by one or more complete wavelengths are said to be in phase. If the distance between points is equal, and they satisfy this condition, they would also be in phase. However, having a whole number multiple of whole wave cycles or wavelengths separating two points is essential for them to be in phase.