Select correct graph, which is plotted between ionization energy of 2nd period elements and their atomic number.

Graph A plotted between ionization energy of 2nd period elements and their atomic number is correct

The overall pattern throughout periods 2 and 3. Ionization energies tend to get stronger with time, on average. 

The outer electrons reside in 2-level orbitals, either 2s or 2p, for the entirety of period 2. The identical 1s2 electrons are used to screen all of them at similar distances from the nucleus.