A composite slab has two layers of different materials with thermal conductivities k1 and k2. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

Answer: Option D

Conductance = $$=\text{k}\times \frac{\text{A}}{l}$$
Where A = slab area, $$l$$ = slab thickness and k = conductivity.
The equation for series conductance is :
$$\text{cond}\left(\text{total}\right)=\frac{\text{con}{\text{d}}_1\times \text{con}{\text{d}}_2}{\text{con}{\text{d}}_1+\text{con}{\text{d}}_2}$$
Because the geometry of the slabs is the same:
$$\text{k}\left(\text{total}\right)=\frac{2\left({\text{k}}_1\times {\text{k}}_2\right)}{{\text{k}}_1+{\text{k}}_2}$$