Glucose is the first cycle of aerobic respiration. It produces two pyruvate molecules, a net gain of two ATP molecules and two $NADH_
_2$
molecules at the end of the cycle. In glycolysis, 2 molecules of ATP are produced during conversion of 1, 3-biphosphoglyceric acid to 3-phosphoglyceric acid and 2-phosphoenol pyruvic acid to pyruvic acid each. However, out of these 4 molecules of ATP, 2 molecules are utilized during conversion of glucose to glucose-6-phosphate and fructose-6-phosphate to fructose-1,6-diphosphate. During the conversion of 2 molecules of 1, 3-diphosphoglyceraldehyde into 2 molecules of 1, 3-diphosphoglyceric acid, 2 molecules of $NADH_
2$
are formed. During aerobic respiration, each $NADH_ 2$
forms 3 ATP and water.
Hence, net gain of ATP molecules in glycolysis is 2 ATP + 6 ATP = 8 ATP.
Thus, the correct answer is '8.'