If Aufbau rule is not followed, K-19 will be placed in :

Since K has atomic number 19,

$1s^{2}2s^{2}2p^{6}3s^{2}3p^6\frac{3d^1}{\uparrow }$ its total will be 19; the final electron will enter the d-orbital.
This makes it a part of the d-block elements.