we Know that gravitational field strength(g) at a point on a planet is equal to gravitational force exerted per unit mass placed at that point.
It Means,
g=F/m
Here,
g=gravitational field strength
F=Gravitational force
m=Mass
Case A
planet force =10 and mass= .5
g1=F/m
g1=10/.5
=100/5
g1=20m/s
case B
F=30 and m=2
therefore g2=30/2
g2=15m/s^2
case C
F=45 and m=3
g3=45/3
=15m/s^2
case D
g4=60/6
g4=10m/s^2
from above results it is clear that the gravitational field strength of planet D is minimum which is 10m/s^2 and gravitational field strength of planet A is maximum which is 20m/s^2