What is the thermal resistance of the wall if thickness ,area,thermal conductivity and temperatures are given?

Question

 A wall of thickness 0.6 m has width has a normal area 1.5 m2 and is made up of material of thermal conductivity 0.4 W/mK. The temperatures on the two sides are 800⁰ C and 100⁰ C. What is the thermal resistance of the wall?                (IES-07)

a. 1 W/K                      b. 1.8 W/K

c. 1 K/W                      d. 1.8 K/W