A long metallic bar is carrying heat from one of its ends to the other end under steady-state. The variation of temperature $\theta$ along the length $x$ of the bar from its hot end is best described by which of the following figure:

In steady state, the rate of heat flow $\frac{dQ}{dt}$ is constant throughout the length of the bar.
$$\frac{dQ}{dt}=-KA\frac{d\theta }{dx}$$
Since $\frac{dQ}{dt}$, $K$, and $A$ are constant:
$$\frac{d\theta }{dx}=-\text{constant}=-m$$
Integrating: $\theta =-mx+c$.This is the equation of a straight line with a negative slope, indicating that the temperature decreases linearly with distance $x$ from the hot end.