smc

Let one of the diagonals of the rhombus be $a$ units long, so the other diagonal is $2a$ units long. That means the area of the rhombus is $\frac{1}{2}\cdot 2a\cdot a=a^2$, so $K=a^2$. From the Pythagorean Theorem, one of the side lengths of the rhombus is $\sqrt{(\frac{a}{2}{)}^2+a^2}=\sqrt{\frac{5a^2}{4}}=\frac{\sqrt{5K}}{2}$, so the answer is $\boxed{\text{None of these are correct}}$.