Let $j$ represent how far Jerry walked, and $s$ represent how far Silvia walked. Since the field is a square, and Jerry walked two sides of it, while Silvia walked the diagonal, we can simply define the side of the square field to be one, and find the distances they walked. Since Jerry walked two sides,
$j = 2$
Since Silvia walked the diagonal, she walked the hypotenuse of a $45$, $45$, $90$ triangle with leg length $1$. Thus,
$s = \sqrt{2} = 1.414...$
We can then take
$\frac{j-s}{j} \approx \frac{2 - 1.4}{2}=0.3 \implies \boxed{\textbf{(A)}\ 30\%}$