**Glycolysis: Oxidation of Glucose to Pyruvate**

Glycolysis is the metabolic pathway that converts glucose into pyruvate, a compound that can be further metabolized to produce energy. It is the first step in both aerobic and anaerobic respiration. Glycolysis occurs in the cytoplasm of cells and does not require oxygen.

**Explanation:**

1. **Glycolysis:** Glycolysis is a series of ten enzyme-catalyzed reactions that occur in the cytoplasm of cells. It is the initial step in the breakdown of glucose and is common to both aerobic and anaerobic respiration.

2. **Conversion of Glucose:** The first reaction in glycolysis involves the conversion of glucose, a six-carbon sugar, into two molecules of glyceraldehyde-3-phosphate, a three-carbon compound.

3. **Energy Investment Phase:** In the energy investment phase, two molecules of ATP are consumed to phosphorylate glucose and convert it into fructose-1,6-bisphosphate. This step requires energy but helps in destabilizing glucose and preparing it for further metabolism.

4. **Energy Payoff Phase:** In the energy payoff phase, glyceraldehyde-3-phosphate is further metabolized to produce ATP and NADH, a high-energy electron carrier. This phase involves a series of reactions that result in the production of two molecules of pyruvate.

5. **Oxidation of Glucose:** Throughout the glycolytic pathway, glucose is oxidized and its carbon atoms are rearranged to form two molecules of pyruvate. This oxidation process generates energy in the form of ATP and reducing equivalents in the form of NADH.

6. **Net Reaction:** The overall net reaction of glycolysis can be summarized as the oxidation of one molecule of glucose to produce two molecules of pyruvate, along with the production of two molecules of ATP and two molecules of NADH.

Therefore, the correct answer is option 'D' - the oxidation of glucose to pyruvate. Glycolysis is the process where glucose is oxidized to pyruvate, releasing energy in the form of ATP and NADH.