Final answer:
All options listed are functions of the respiratory system, including gas exchange, pH/acid-base balance, and speech/vocalization. correct option is A.
Step-by-step explanation:
The respiratory system has several important functions such as gas exchange (O2 and CO2), maintaining pH/acid-base balance, and facilitating speech/vocalization. Therefore, option D, stating that all of the above are functions, is the correct answer as none of the listed functions are 'NOT a respiratory system function'.