Which of the following is the electron configuration of an electrically neutral atom of argon (Ar)?

(a) 1s² 2s² 2p5
(b) 1s² 3d¹⁰ 4f⁷
(c) 1s² 2s² 2p⁶ 3s² 3s⁶
(d) 1s² 2s1