Ionization energy increases left to right across a period because 

(a) It does not. Ionization energy decreases left to right across a period.

(b) As you go down a group, atoms increase in size due to increasing values of n.

(c) Atoms shrink in size, resulting in a more positive nucleus and a tighter hold on electrons.

(d) Atoms shrink in size, resulting in increasing attraction between the electrons and the nucleus.