Equation for overall heat transfer coefficient
This is the heat equation analogue to Ohm's law, in which resistances are additive (in series), and hence the inverse conductances are additive. 

The way to reason is that the full equation (1) for the heat conductivity (1) contains three terms: one for the interior of the tube, one for the tube wall, and a third for the exterior. When you look at the profile, it becomes evident that, since temperatures immediately adjacent inside and outside the tube are equal, the tube has infinite heat conductance ($k=\infty$).

The right answer is therefore 3.