0.69 mol / 25 eq = 27.6 mmol O2
0.20 mol / 2 eq = 100 mmol octane
therefore...
a) oxygen is the limiting reagent
b) 27.6 mmol x 18 eq = 497 mmol or 0.0497 moles H2O produced.
c) 27.6 mmol x 2 eq = 55.2 mmoles octane consumed
200 mmol - 55.2 mmol = 144.8 mmol or 0.145 moles octane remaining.