number of ATP molecules which can be built on complete oxidation of pyruvic acid

1. 6
2. 2
3. 15
4. 30

please explain

15

1 Glycolysis yield 8 ATP 

2 Oxidative decarboxylation NADH = 3 ATP 

3 Krebs cycle 

a oxidation of isocitric acid to oxalo succinic acid is 3ATP 

b oxidation of ketoglutaric acid to succinyl coA is 3 ATP 

c oxidation of succinic acid to fumaric acid is 2 ATP 

d oxidation of malic acid to oxaloacetic acid is 3 ATP

e conversion of succinylCOA to succinic acid is 1 ATP

Thus the complete oxidation of pyruvic molecule yields 15 ATP

one glucose produces 2 pyruvic acid so its 15x2 = 30 + 8 ATP from glycolysis so total is 38 for complete oxidation of glucose.