Answer
We can rank the choices according to the magnitude of the normal force acting on the block from the plane:
$d \gt c \gt a \gt b$
Work Step by Step
The magnitude of the normal force acting on the block from the plane is equal to the sum of the component of the block's weight directed into the plane and the component of the force directed into/out of the plane.
We can find an expression for the magnitude of the normal force $F_N$ for each applied force:
$a$
$F_N = mg~cos~30^{\circ}- F~sin~30 = 0.866~mg-0.5~F$
$b$
$F_N = mg~cos~30^{\circ}- F~cos~30 = 0.866~mg-0.866~F$
$c$
$F_N = mg~cos~30^{\circ}+ F~sin~30 = 0.866~mg+0.5~F$
$d$
$F_N = mg~cos~30^{\circ}+ F~cos~30 = 0.866~mg+0.866~F$
We can rank the choices according to the magnitude of the normal force acting on the block from the plane:
$d \gt c \gt a \gt b$
