Final answer:
When muscle cells lack oxygen but need more ATP, they convert pyruvate into lactic acid. This process allows glycolysis to continue and is a form of anaerobic respiration.
Step-by-step explanation:
When muscle cells lack oxygen but need to generate more ATP, they convert pyruvate into lactic acid, which is the correct answer to the question, A) Lactic acid (product). This process is known as anaerobic respiration or fermentation and occurs during intense exercise when muscles require energy rapidly, but the oxygen supply is inadequate. The pyruvate is converted to lactic acid to regenerate NAD+, allowing glycolysis to continue producing ATP, albeit not as efficiently as aerobic respiration. The lactic acid can lead to muscle fatigue and is eventually transported to the liver where it can be turned back into pyruvate or glucose through the Cori cycle, especially once oxygen is readily available again.