Answer
b=d>a=c
Work Step by Step
This is because the formula for the potential energy $U=\frac{Kq_{1}q_{2}}{r}$. As you can see, this is directly proportional to each charge and to $\frac{1}{r}$. Calling each plus sign a charge of +1 we find that:
a. $U=\frac{1*1*K}{r}=\frac{K}{r}$
b. $U=\frac{2*1*K}{r}=\frac{2K}{r}$
c. $U=\frac{2*1*K}{2r}=\frac{K}{r}$
d. $U=\frac{2*2*K}{2r}=\frac{2K}{r}$
From this we see that b=d>a=c
