a) `1s^(2) 2s^(2) 2p^(3) implies `

(b) `1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(6) 3d^(2) implies Z = 20`
c) Electronic configuration of He is `1 s^(2) implies `

Which one contradicts the Aufbau rule ?

Explanation

1) 'a' contradicts the Hund's rule. 2) In the given electron block diagram

`2p_(x)` was filled with two electrons, without filling of `2p_(z)` orbital with single electron
3) According to Hund's rule electron pairing in orbitals starts only when all available empty orbitals of the same energy are singly occupied.