Answer
We can rank the arrangements according to the magnitude of the net electrostatic force on the particle with charge $+Q$:
$a = d \gt b = c$
Work Step by Step
We can write an expression for the force between two charged particles:
$F = \frac{q_1~q_2}{4\pi~\epsilon_0~r^2}$
Note that an electron and a proton have the same magnitude of charge.
In all four cases, note that the vertical component of the force on the $+Q$ charge has the same magnitude.
In arrangement (a), the horizontal components of the forces on the $+Q$ charge due to the other two particles are in the same direction. Thus the horizontal component of the force on the $+Q$ charge is a maximum.
In arrangement (b), the horizontal components of the forces on the $+Q$ charge due to the other two particles are in the opposite direction. Thus the horizontal component of the force on the $+Q$ charge is not a maximum.
In arrangement (c), the horizontal components of the forces on the $+Q$ charge due to the other two particles are in the opposite direction. Thus the horizontal component of the force on the $+Q$ charge is not a maximum.
In arrangement (d), the horizontal components of the forces on the $+Q$ charge due to the other two particles are in the same direction. Thus the horizontal component of the force on the $+Q$ charge is a maximum.
We can rank the arrangements according to the magnitude of the net electrostatic force on the particle with charge $+Q$:
$a = d \gt b = c$
