When lactic acid is formed during fermentation ?

Explanation

### Step-by-Step Solution:

1. **Understanding Fermentation**: Fermentation is a metabolic process that occurs in the absence of oxygen (anaerobic conditions). This process allows organisms to convert sugars into energy without using oxygen.

2. **Conditions for Lactic Acid Formation**: Lactic acid is specifically produced during fermentation when oxygen is limited or not available. This typically occurs in certain bacteria and in animal muscle cells during intense exercise.

3. **Role of Pyruvic Acid**: During fermentation, glucose is broken down to produce pyruvic acid. In the absence of oxygen, this pyruvic acid undergoes further conversion.

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