The effective nuclear charge felt by the 4s electron in potassium atom is .

The correct option is B 2.2
The electronic configuration of potassium (K) is $1s^{2}2s^{2}2p^{6}3s^3{p}^{6}4s^1$. There are four shells in potassium.
$(Z_{eff}{)}_{4s}$ = $Z-\sigma$

$\sigma =(0.85\times numberofelectronsinthe(n-1)thshell+1.00\times totalnumberofelectronsintheinnershell)$

$=0.85\times 8+1.00\times 10$

$=16.80$

Hence, $Z_{eff}=Z-\sigma$
$=19-16.80$
$=2.20$