Final answer:
In M's question, they are inquiring about the control of voice pitch. The pitch is changed by relaxing or tightening the vocal cords located in the larynx, specifically the space called the glottis.
Step-by-step explanation:
In M's question, they are asking about how humans change the pitch of their voice. The correct answer is a. Relaxing or tightening their glottis.
Humans can control the pitch of their voice through the use of the larynx, where the vocal cords, or vocal folds, are located. The glottis is part of the larynx; it is the space between the vocal cords. When we change the tension of the vocal cords by relaxing or tightening them, the pitch of the voice is altered. For a higher pitch, the cords are tightened, which makes them vibrate faster. Conversely, for a lower pitch, the cords are relaxed, allowing slower vibration.
It's important to note that the uvula has a role in speech, but it does not directly contribute to the pitch of the voice. Therefore, option b is incorrect as it relates to the uvula.