To determine the unit of thermal conductivity, we need to understand the formula for heat conduction.$\\$The rate of heat transfer $(Q)$ through a material is given by Fourier's law of heat conduction:$\\ Q = -k \cdot A \cdot \frac{dT}{dx} \\$Where:$\\$• $Q$ is the heat transfer per unit time (J/s or W)$\\$• $k$ is the thermal conductivity$\\$• $A$ is the cross-sectional area (m$^2$)$\\$• $\frac{dT}{dx}$ is the temperature gradient (K/m)$\\$Rearranging the formula to solve for $k: \\ k = -\frac{Q}{A \cdot \frac{dT}{dx}} \\$Substituting the units:$\\ k = \frac{W}{m^2 \cdot \frac{K}{m}} \\$Simplifying the units:$\\ k = \frac{W \cdot m}{m^2 \cdot K} \\ k = \frac{W}{m \cdot K} \\$Therefore, the unit of thermal conductivity is $$W m$^{-1}$ K$^{-1}. \\$This corresponds to Option 4.