Answer is : (a) O, F and Ne
As O, F and Ne belong to same period, i.e. 2nd period, the ionisation energies increases on moving from left to right. This increase in ionisation energy is due to decrease in the atomic radii across a period. Thus, the first ionisation enthalpies for O, Fe and Ne are 1314, 1680 and 2080, respectively.
So, the correct sequence of the element is O, F and Ne.