equivalent thermal conductivity of the wall

Question

The equivalent thermal conductivity of the wall as shown in the figure is 

a)$\frac{k_1+k_2}{2}$

b)$\frac{k_1k_2}{k_1+k_2}$

c)$\frac{2k_1k_2}{k_1+k_2}$

d)$\sqrt{k_1k_2}$