The correct answer is: A. Frequency of vibration.
The frequency of vibration of a stringed instrument is the number of times the string vibrates per second. The higher the frequency, the higher the pitch of the sound. The length of the string, the tension of the string, and the mass of the string all affect the frequency of vibration.
The amplitude of vibration is the maximum displacement of the string from its equilibrium position. The higher the amplitude, the louder the sound. The amplitude of vibration is affected by the force that is applied to the string and the stiffness of the string.
The waveform of the sound is the shape of the graph of the sound pressure versus time. The waveform of the sound is affected by the way that the string vibrates and the way that the sound is transmitted through the air.
The quality or tone of a musical sound is determined by the frequency, amplitude, and waveform of the sound. The frequency of vibration is the most important factor in determining the pitch of the sound. The amplitude of vibration is the most important factor in determining the loudness of the sound. The waveform of the sound is the most important factor in determining the timbre of the sound.