The correct answer is option 2: its amplitude.
The loudness of sound refers to how loud or soft a sound is perceived by a person. It is determined by the amplitude or intensity of the sound wave. Amplitude is the maximum displacement of particles in a medium as the sound wave passes through it.
Option 1, frequency, refers to the number of complete oscillations or vibrations of a sound wave in a given time. Although frequency does affect the pitch of a sound (how high or low it sounds), it is not directly related to the loudness of the sound.
Option 3, speed, refers to how fast the sound wave is traveling through a medium. While the speed of sound does influence the arrival time of a sound wave, it does not have a direct impact on the loudness of the sound.
Option 4, pitch, refers to the perceived frequency of a sound. Like frequency, it affects how high or low a sound appears, but it does not determine the loudness of the sound.
Therefore, the correct option that is related to the loudness of sound is option 2, its amplitude.