The correct decreasing order of first ionisation enthalpies of five elements of the second period is

As we move along the period, the atomic size decreases due to increase in nuclear charge. Therefore, it is more difficult to remove electron from an atom. Hence the sequence of first ionization enthalpy in decreasing order is
F>N>C>B>Be
But ionization enthalpy of boron is less as compared to beryllium because first electron in boron is to be removed from p-orbital while in beryllium, is to be removed from s-orbital.

As s - orbital is closer to nucleus in comparison to p-orbital thus energy required to remove an electron from 

s - orbital is greater.