Density $\rho$ is defined as $\rho = \frac{Mass}{Volume}$. For a cube of side $L$, the volume $V = L^3$. Thus, $\rho = \frac{M}{L^3}$. The relative error in density is given by $\frac{\Delta \rho}{\rho} = \frac{\Delta M}{M} + 3\frac{\Delta L}{L}$. Converting this to percentage error: Percentage error in $\rho = (\text{Percentage error in } M) + 3 \times (\text{Percentage error in } L)$. Substituting the values: $3\% + 3(2\%) = 3\% + 6\% = 9\%$. Option 5% is incorrect because it simply adds the errors without accounting for the cubic relationship of length. Option 7% is incorrect calculation, and 11% is irrelevant.