Assertion: The ${19}^{th}$ electron in potassium atom enters into the $4s$ orbital and not into the $3d$ orbital.
Reason: $(n+l)$ rule is followed for determining the orbitals of the lowest energy state.

The correct option is A If both Assertion and Reason are true and the Reason is a correct explanation of the Assertion
$(n+l)$ value:
$4s=4+0=43d=3+2=5$
According to Aufbau principle, the unfilled shell with lowest $(n+l)$ value will be filled first. Therefore, $4s$ orbital is filled first then $3d$ orbital.
$K:1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
Option a is correct