Question

lactic acid fermentation is anaerobic. this occurs when our body switched from aerobic respiration to anaerobic respiration. why do your legs hurt after running for a while?
a buildup of lactic acid
a buildup of atp
too many mitochondria

Explanation:

When running, if oxygen is insufficient, the body switches to anaerobic respiration (lactic acid fermentation). Lactic acid fermentation produces lactic acid. A buildup of lactic acid in muscle cells (like in legs) causes muscle soreness or pain. ATP is energy and its buildup doesn't cause pain, and more mitochondria would support aerobic respiration, not cause pain. So the correct reason is lactic acid buildup.

Answer:

a buildup of lactic acid