Scandium is a d-Block element with atomic number 21. So, it has 21 protons and 21 electrons in neutral state. Electronic Configuration for Sc is,
                               1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹
Also we know that, 
Electronic configuration of Argon with atomic number 18 is,
                              [Ar]  =  1s², 2s², 2p⁶, 3s², 3p⁶
Now comparing both configurations,
                                            [Ar], 4s², 3d¹
This is condensed electronic configuration of Scandium.