What causes male voices to crack during puberty?

The correct option is C

Vocal cords thickening

The rising levels of testosterone in males during puberty trigger a lot of changes in the body like an increase in musculature, facial and pubic hair, etc. During puberty, testosterone also causes a boy’s larynx to get bigger. The vocal cords also lengthen and thicken in response to testosterone. As the larynx and vocal cords are going through this growth phase, it causes the voice to “crack" and eventually males have deeper voices.