$\frac{{dQ}}{{dt}} = \frac{{kA\left( {{\theta _1} - \theta } \right)}}{x}$
$ \Rightarrow {\theta _1} - \theta = \frac{x}{{kA}}\frac{{dQ}}{{dt}} \Rightarrow \theta = {\theta _1} - \frac{x}{{kA}}\frac{{dQ}}{{dt}}$
Where $\theta _1$ is the temperature of hot and $\theta $ is temperature at a distance $x$ from hot end.
The above equation can be graphically represented by option $(a)$.