To find the thermal conductivity of the composite rod, we need to consider the rods in parallel.$\\$Given:$\\$• Thermal conductivities of rods A and B are $K_1$ and $K_2$ respectively.$\\$• Both rods have the same length $d$ and are at temperatures $T_1$ and $T_2. \\$When rods are in parallel, the effective thermal conductivity $K_{eq}$ is given by:$\\ K_{eq} = \frac{K_1 + K_2}{2} \\$This is because the heat flow is distributed equally across the cross-sectional areas of both rods.$\\$Therefore, the thermal conductivity of the composite rod is $\frac{K_1 + K_2}{2}. \\$This corresponds to Option 4.