If the Aufbau rule is not followed, Ca-20 will be placed in the ...........block.

If the Aufbau principle had not been followed, the filling of the 3d orbital would have been prior to the filling of the 4s orbital and the electronic configuration of Ca would have been $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}3d^2$ and it would have been placed in the d-block.

Hence, the correct option is C.