Final answer:
During puberty, the voice of 14-15-year-old boys deepens due to the growth of their vocal cords stimulated by the hormone testosterone.
Step-by-step explanation:
During puberty, several changes occur in the male body, including a deepening of the voice.
This is primarily caused by the growth of the vocal cords.
The hormone testosterone stimulates the growth of the larynx and the lengthening and thickening of the vocal folds, resulting in a lower pitch of the voice.
Therefore, the correct answer is b) Growth of the vocal cords.