Answer
We can rank the situations according to the magnitude of the net electric field at that central point:
$a \gt c \gt b = d$
Work Step by Step
(a) By the right hand rule, the electric fields due to the four rods are all directed in the same direction.
(b) By the right hand rule, the electric fields due to the $-4$ rods cancel out and the electric fields due to the $+2$ rods cancel out. The net electric field is zero.
(c) By the right hand rule, the electric fields due to the $+8$ rods cancel out, while the electric fields due to the $+2$ and $-2$ rods only are directed in the same direction.
(d) By the right hand rule, the electric fields due to the $-6$ rods cancel out and the electric fields due to the $+5$ rods cancel out. The net electric field is zero.
We can rank the situations according to the magnitude of the net electric field at that central point:
$a \gt c \gt b = d$
