During which stage in the complete oxidation of glucose are the greatest number of ATP molecules formed from ADP?

• Conversion of pyruvic acid to acetyl Co-A

• Electron transport chain

• Glycolysis

• Krebs cycle

B.

Electron transport chain

In glycolysis, 4 ATP and 2 NADH₂ molecules are formed. These 2 NADH₂ molecules go to electron transport chain.

In oxidative decarboxylation no ATP molecules are formed but two molecules of NADH₂ are formed from two molecules of pyruvate. And both NADH₂ molecules go to ETC (Electron Transport Chain). 

In Krebs cycle 2 ATP, 6NADH₂ and 2FADH₂ molecules are formed from two molecule of acetyl Co-A. These NADH₂ and FADH₂ go to ETC.

In ETC all NADH₂ and FADH₂ pass to electron carriers and yield 3 ATP and 2 ATP molecules per NADH₂ and FADH₂ respectively. Thus,
$\to$4 ATP are formed in glycolysis
$\to$2 ATP in Krebs cycle and
$\to$34 ATP from electron transport chain
$\to$40 ATP
$\to$2 ATP molecules are used during glycolysis.
So, net gain of ATP molecules during one complete oxidation of a glucose molecule is 38 ATP.