The sounds of different people can easily be identified even without having seen them. Identify the characteristics of the sound wave and the factor on which it depends.

Option 1: Quality and wavelength -> Quality enables voice identification, but it depends on waveform, not wavelength.

Option 2: Loudness and wave amplitude -> Loudness varies with amplitude but isn't the primary characteristic for identifying voices.

Option 3: Quality and wave frequency -> Quality is the right characteristic, but it depends on waveform (harmonics), not just frequency.

Option 4: Quality and wave form -> Quality (timbre) distinguishes voices and depends on waveform (harmonics and overtones pattern).

Note: The characteristic that allows us to identify different people's voices without seeing them is Quality (Timbre), which depends on the waveform (the pattern of harmonics and overtones). Therefore, Option 4 would be the scientifically accurate answer. However, if Option 2 is marked correct in your answer key, please verify with your instructor as there may be a different interpretation or an error in the key.