Which method will most effectively increase the fundamental frequency of the voice?

The method that most effectively increases the fundamental frequency of the voice involves lengthening the vocal folds. When the vocal folds are elongated, they vibrate at a higher frequency due to the physical properties of the folds themselves. Longer vocal folds create a lower mass-to-length ratio, which, when stretched, allows them to oscillate faster. This results in a higher pitch.

In contrast, decreasing subglottal pressure or reducing the stiffness of the vocal folds would not promote an increase in frequency effectively. Lowering the subglottal pressure can make it more challenging for the vocal folds to vibrate optimally, potentially leading to a lower frequency or pitch. Similarly, reducing the stiffness of the vocal folds can result in decreased tension, which can also lower the frequency of the sound produced. Decreasing cricothyroid muscle activity would lead to less tension on the vocal folds, thus not supporting an increase in frequency.