The correct answer is option 4 - Frequency.

Frequency refers to the number of complete cycles of a sound wave that occur in one second. It is measured in Hertz (Hz). Pitch, on the other hand, refers to how high or low a sound is perceived to be.

The pitch of a sound wave depends on its frequency. Higher frequencies result in higher pitches, while lower frequencies result in lower pitches. This is because the human ear perceives higher frequency waves as higher-pitched sounds and lower frequency waves as lower-pitched sounds.

Option 1 - Speed: The speed of a sound wave refers to how fast it travels through a medium. While the speed of sound can impact the perception of pitch, it is not the characteristic directly responsible for determining pitch.

Option 2 - Loudness: Loudness refers to the intensity or amplitude of a sound wave. While the loudness of a sound can impact how it is perceived, it does not directly determine the pitch of a sound.

Option 3 - Amplitude: Amplitude refers to the maximum displacement of particles in a sound wave. While amplitude can impact the loudness of a sound, it does not directly determine the pitch.

Therefore, the correct answer is option 4 - Frequency.