c
Electronic configuration of \(Ca\) is \(1 s ^2 \,2 s ^2\, 2 p ^6 \,3 s ^2\, 3 p ^6\, 4 s ^2\).

Filling of \(3 d\) orbital starts after the filling of \(4 s\) orbitals. If the Aufbau principle had not been followed, the filling of \(3 d\) orbital would have been prior to filling of \(4 s\) orbital and the electronic configuration of \(Ca\) would have been \(1 s ^2\, 2 s ^2 \,2 p ^6\, 3 s ^2\, 3 p ^6 \,3 d ^2\) and it would have been placed in the \(d\)-block.