Final answer:
During glycolysis, four hydrogen atoms are released as two molecules of NADH are formed, each carrying a pair of electrons from the oxidation steps. The correct answer is D. four.
Step-by-step explanation:
In the biological process of glycolysis, each molecule of glucose (a 6-carbon sugar) is broken down into two molecules of pyruvate (a 3-carbon compound). During this conversion, a total of four ATP molecules are synthesized, but since two ATP molecules are consumed in the early steps, the net gain of ATP is two.
Regarding the release of hydrogen atoms, the oxidation of glyceraldehyde-3-phosphate results in the reduction of NAD+ to NADH. This means that two NADH molecules are produced, each carrying a pair of electrons which were originally associated with hydrogen atoms. Therefore, for each glucose molecule undergoing glycolysis, four hydrogen atoms are released overall (two from each of the two 3-carbon compounds).
Thus, the correct answer to the question is D. four.