The correct option is C 1s2, 2s2, 2p6
Screening effect is the effect produced by intervening electrons between nucleus and valence electrons. They shield the nucleus from valence electron and effective nuclear charge decreases if there is less shielding effect. Valence electrons are attracted by nucleus and repelled by other electrons.
As Electronic Configuration of Na atom is: 1s2 2s2 2p6 3s1
So, Intervening electrons between nucleus and valence electrons, that are responsible for screening effect are 1s2 2s2 2p6.