The correct answer is: waves with the same frequency can have different timbres. Let's explain why the other alternatives are incorrect: a) When a sound wave passes from one medium to another (from air to water, for example), the frequency of the wave remains constant. What changes is the propagation speed and, consequently, the wavelength. b) In the refraction of sound waves, there is a change in the wavelength, as the wave's speed changes when passing from one medium to another. The frequency, however, remains the same. d) Sound waves do not propagate in a vacuum because they require a material medium (such as air, water, or solids) to propagate. In a vacuum, there are no particles to transmit sound vibration. e) The frequency of a sound wave is related to the pitch of the sound: the higher the frequency, the sharper the sound; the lower the frequency, the deeper the sound. Therefore, the correct alternative is (c), as timbre is a characteristic of sound that allows us to distinguish two sound sources emitting sounds at the same frequency. Timbre is determined by the shape of the sound wave, which is influenced by the nature of the sound source and the presence of harmonics or overtones.