a b c d

b

$ R = \frac{\rho l}{A}$ $ 3 =  \frac{\rho l}{A}$ $\Rightarrow \frac{A}{l} = \frac{\rho}{3}$ $\Rightarrow V = Al = 3$ $\text{Multiplying these two equations }A^2 = \rho $ $ \Rightarrow A = \sqrt \rho $ $\Rightarrow l = \frac{3}{\sqrt \rho}$