When the pitch of sound increases, the correct answer is option 4 - frequency, which refers to the number of waves produced per unit of time.

Intensity, option 1, refers to the amplitude or strength of the sound wave. It determines the loudness of the sound, but not the pitch.

Loudness, option 2, is the perception of the intensity of sound by our ears. It also depends on the amplitude of the wave, not the pitch.

Wavelength, option 3, refers to the distance between two consecutive points in a wave. It is inversely proportional to the frequency of the sound wave and not directly related to the pitch.

Frequency, option 4, determines the pitch of a sound. It is the number of complete cycles of a wave that occur in one second, measured in hertz (Hz). As the frequency increases, the pitch becomes higher, and as it decreases, the pitch becomes lower.

Therefore, when the pitch of sound increases, the frequency of the sound wave increases.