**Explanation:**

The correct answer is option **B) Wavelength**.

Let's discuss each characteristic of a musical sound in detail:

**a) Pitch:** Pitch refers to the highness or lowness of a sound. It is determined by the frequency of the sound wave. Higher frequencies produce higher pitches, while lower frequencies produce lower pitches. For example, a high-pitched sound would be the sound produced by a whistle, while a low-pitched sound could be the sound produced by a bass guitar.

**b) Wavelength:** Wavelength is a characteristic of a sound wave, but it is not specifically related to musical sounds. Wavelength refers to the distance between two consecutive points of a sound wave that are in phase with each other. It is inversely related to frequency, meaning that shorter wavelengths correspond to higher frequencies. However, wavelength is not typically used to describe musical sounds.

**c) Quality (Timbre):** Quality, also known as timbre, refers to the characteristic sound of a particular instrument or voice. It allows us to distinguish between different musical instruments or voices producing the same pitch. For example, a piano and a guitar may both play the same note, but they will have different qualities or timbres.

**d) Loudness:** Loudness refers to the perceived volume or intensity of a sound. It is determined by the amplitude of the sound wave. Higher amplitudes produce louder sounds, while lower amplitudes produce softer sounds. For example, a loud sound would be the sound produced by a drum, while a soft sound could be the sound produced by a whisper.

In conclusion, wavelength is not a characteristic of a musical sound. While it is a characteristic of a sound wave, it is not typically used to describe musical sounds. The other characteristics, pitch, quality, and loudness, are important aspects of musical sounds.