Explanation

(c) Rate of loss of heat $(R) \propto$ temperature difference
$\Rightarrow R \propto \left( \theta - \theta_{0} \right) \Rightarrow R = k\left( \theta - \theta_{0} \right) = k\theta - k\theta_{0}\ \lbrack k =$ constant]
On comparing it with $y = mx + c$ it is observed that, the graph between $R$ and $\theta$ will be straight line with slope $= k$ and intercept $= - k\theta_{0}$