Final answer:
In the absence of oxygen, pyruvic acid enters anaerobic respiration, a fermentation process where it can be converted into lactic acid.
Step-by-step explanation:
When no oxygen is present, pyruvic acid will enter into a type of respiration known as anaerobic respiration. In anaerobic respiration, pyruvate undergoes a process called fermentation. During fermentation, pyruvate can be converted into products like lactic acid. This process is crucial for the recycling of NADH back into NAD+, which is necessary for glycolysis to continue producing ATP, albeit at a lower yield than aerobic respiration. Anaerobic respiration is typical in muscle cells during intense exercise when oxygen supply is limited and in organisms that thrive in environments devoid of oxygen.
Therefore, the correct answer to the question is 2) Anaerobic respiration.