What is the primary function of both alcohol fermentation and lactic acid fermentation?

Oxidize NADH to NAD+

Explanation:

Fermentation is of two types alcoholic fermentation and lactic acid fermentation both occurs without oxygen. In alcoholic fermentation, glucose is first converted into two pyruvic acid molecule and pyruvic acid is then converted into lactic acid by oxidizing NADH to NAD+.

In lactic acid fermentation glucose is first break into 2 pyruvic and than this pyruvic acid is converted into lactic acid by oxidizing 2 NADH molecules into NAD+. Therefore the correct answer is to oxidize NADH to NAD+.