The pitch of a note is the same as its frequency.
Remember that te timbre depends on the shape and other properties of the vibrating object. The loudness is related to the amplitude of the wave, and the wavelength of sound is inversely proportional to the frequency.
On the other hand, the frequency of the sound of a string is proportional to the square root of the tension in the string. Then, if the tension increases (if the string is tightened), the frequency will also increase (and if the frequency increases, the wavelength decreases).
Therefore, the parameter that increases when a string of a stringed instrument is tightened, is the pitch.
Therefore, the answer is:
