1) write a balanced equation:
C6H12O6 + 6O2 --------> 6 H2O + 6CO2
(1 mol) (6 mol) (6 mol) (6 mol)
20g C6H12O6 (glucose)
2) convert substance being combusted into moles
C6H12O6 : Molecular weight = 180 g/mol
So, 20g/180g/mol = 1/9 mol of glucose
1 mol C6H12O6 : 6 mol H2O
1/9 mol C6H12O6 : X mol H2O
x = 6/9 = 2/3 mol H2O
So, the correct answer is A