For an element, with the electronic configuration $1s^{2}2s^{2}2p^{6}3s^2$, one of the 3s electrons will be shielded from the nuclear charge most effectively by a:

The electronic configuration is,

$1s^{2}2s^{2}2p^{6}3s^2$

Here one of the 3s electrons will be shielded from the nuclear charge most effectively by a is 1s electron, because 1s electron are mostly shilded of an atom.