Answer
The ratio of $F_{ca}$ to $F_{ba}$ is $\frac{1}{2}$
Work Step by Step
Let $L$ be the length of each side of the square.
We can find an expression for $F_{ba}$:
$F_{ba} = \frac{k~Q^2}{L^2}$
We can find an expression for $F_{ca}$:
$F_{ca} = \frac{k~Q^2}{(\sqrt{2}~L)^2} = \frac{k~Q^2}{2~L^2}$
We can find the ratio of $F_{ca}$ to $F_{ba}$:
$\frac{F_{ca}}{F_{ba}} = \frac{\frac{k~Q^2}{2~L^2}}{\frac{k~Q^2}{L^2}} = \frac{1}{2}$
