What does it mean for waves to be in phase?

When waves are described as being in phase, it means that their peaks and troughs occur at the same time, which corresponds to option C. In a practical sense, this implies that the waves oscillate together, reaching their maximum and minimum points synchronously. This synchronization can lead to constructive interference when the waves overlap, resulting in a higher amplitude than either wave alone.

In contrast, having different frequencies would mean the waves oscillate at different rates, which prevents them from being in phase. Similarly, if the peaks and troughs occurred at different times, you would have waves that are not in phase, leading to potential cancellation or destructive interference. Lastly, if the waves are traveling in opposite directions, they could still be in phase if their peaks and troughs line up; however, without this line-up, they can interfere with each other in ways that would prevent them from being considered in phase. Thus, being in phase specifically refers to the timing of their peaks and troughs, confirming why option C is the correct choice.