Lactic acid is formed by the process of incomplete oxidation.

Explanation:
Lactic acid is a type of organic acid that is produced in the body during certain metabolic processes. It is formed when glucose is metabolized in the absence of oxygen, a process called anaerobic glycolysis. Here is a detailed explanation of the process:

1. Glycolysis:
Glycolysis is the initial step in the breakdown of glucose, which occurs in the cytoplasm of cells. During glycolysis, one molecule of glucose is converted into two molecules of pyruvate. This process yields a small amount of ATP (adenosine triphosphate), which is a source of energy for cells.

2. Aerobic Respiration:
Under normal aerobic conditions, pyruvate is further oxidized in the mitochondria through a series of reactions called the Krebs cycle or the citric acid cycle. This process generates a large amount of ATP and carbon dioxide as byproducts.

3. Anaerobic Respiration:
In the absence of oxygen, pyruvate cannot enter the Krebs cycle. Instead, it undergoes a process called fermentation. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation.

4. Lactic Acid Fermentation:
During lactic acid fermentation, pyruvate is converted into lactic acid. This process occurs in various tissues such as muscle cells when the demand for energy exceeds the oxygen supply (e.g., during intense exercise). The conversion of pyruvate into lactic acid regenerates NAD+ (nicotinamide adenine dinucleotide), which is necessary for glycolysis to continue. However, lactic acid fermentation is an inefficient process, as it only produces a small amount of ATP compared to aerobic respiration.

Conclusion:
In summary, lactic acid is formed by the process of incomplete oxidation, specifically during lactic acid fermentation. This process occurs when glucose is metabolized in the absence of oxygen, leading to the production of lactic acid as a byproduct.