The correct option is B 20 N/C
Given:
Thickness of sheet, d=4 cm=0.04 m
Distance of point P from mid-line of sheet, x=6 cm=0.06 m;
Charge density, ρ=9 nC/m3;
Here, x>d2
We know that the electric field for a point outside the thick sheet is given by,
E=σ2ϵ0=ρd2ϵ0
⇒E=9×10−9×0.042×9×10−12=20 N/C
So, option (b) is correct.