During lactic acid fermentation, pyruvic acid is reduced to lactic...

During lactic acid fermentation, pyruvic acid is reduced to lactic acid by the enzyme lactate dehydrogenase.

The reducing agent for this reaction is $\text{NADH} + \text{H}^+$, which is produced during the oxidation steps of glycolysis.

In this process, $\text{NADH} + \text{H}^+$ donates electrons and a proton to pyruvic acid, getting reoxidized to $\text{NAD}^+$, which is then reused in glycolysis.

Answer: $\text{NADH} + \text{H}^+$ produced during glycolysis.