I am not sure if one of the answers can be called "the correct one".
From what I remember IUPAC doesn't define precisely how the balanced reaction should look like. It is quite common to follow the convention of using the lowest natural numbers in balanced reaction equation, so I wouldn't call your answer wrong.
At the same time sometimes it is convenient to write reaction using fractional numbers. When you have a list of several alkanes writing their combustion reactions in such a way they all start with a single molecule (even if this means fractional coefficients of oxygen) makes comparisons (of amount of oxygen required, of molar combustion heat) easier.