Answer
We can rank the situations in order of the magnitude of the electric force on $Q_1$, from largest to smallest:
$a = b \gt d \gt c = e$
Work Step by Step
We can write a general expression for the electric force exerted on charge $q_1$:
$F = \frac{k~q_1~q_2}{r^2}$
Let $q = 1~\mu C$
Let $r = 1~m$
In each case, we can find an expression for the electric force exerted on charge $Q_1$.
(a) $F = \frac{k~Q_1~Q_2}{d^2} = \frac{k~(q)(2q)}{(r)^2} = 2\times \frac{k~q^2}{r^2}$
(b) $F = \frac{k~Q_1~Q_2}{d^2} = \frac{k~(2q)(-1q)}{(r)^2} = -2\times \frac{k~q^2}{r^2}$
(c) $F = \frac{k~Q_1~Q_2}{d^2} = \frac{k~(2q)(-4q)}{(4r)^2} = -\frac{1}{2}\times \frac{k~q^2}{r^2}$
(d) $F = \frac{k~Q_1~Q_2}{d^2} = \frac{k~(-2q)(2q)}{(2r)^2} = -1\times \frac{k~q^2}{r^2}$
(e) $F = \frac{k~Q_1~Q_2}{d^2} = \frac{k~(4q)(-2q)}{(4r)^2} = -\frac{1}{2}\times \frac{k~q^2}{r^2}$
We can rank the situations in order of the magnitude of the electric force on $Q_1$, from largest to smallest:
$a = b \gt d \gt c = e$
