1. Two waves have the same frequency. Which wave characteristic must

also be the same for both waves?

Wavelength is the wave characteristic which is same for both waves.

Explanation:

Two waves having same frequency also have same time period, amplitude, speed and wavelength.

If two waves which are travelling are travelling with same frequency it means that they are having same time period and amplitude. If the speed of waves and frequency are same then wavelength of the wave is also same to the frequency.

Therefore wavelength is the characteristic of a wave is same to speed and frequency.