Which of the following fermentation methods can occur in animal skeletal muscles?
lactic acid fermentation
alcohol fermentation
mixed acid fermentation
propionic fermentation

The correct answer would be lactic acid fermentation.

Explanation:

In humans, specially in muscle cells, lactic acid fermentation takes place during high intensity exercise. During exercise when oxygen supply fails to meet the demands of muscle, they undergo lactic acid fermentation instead of cellular respiration.

During this process, the pyruvate (end product of glycolysis) is converted into the lactate with the help of enzyme lactate dehydrogenase.

It regenerates NAD⁺ lost in the glycolysis and thus, helps in continuous working of glycolysis.

a "theory" a suggestion that is not 100 % accurate, or a figure. some people say its a theory because they of all the passed theory's about one subject. it depends sometimes theory might be accurate and sometimes they are.