$\text{C(s)}+2{\text{H}}_2\text{(g)}\to {\text{CH}}_4\text{(g)};\Delta H=-74.8{\text{kJ mol}}^{-1}$. Which of the following diagrams gives an accurate representation of the above reaction? [$R\to$ reactants; $P\to$ products]

The reaction is exothermic ($\Delta H=-74.8{\text{kJ mol}}^{-1}$), meaning the energy of the products ($P$) must be lower than the energy of the reactants ($R$).Diagram (1) shows $R$ lower than $P$ (endothermic).Diagram (2) shows $R$ and $P$ levels, but labels $P$ as higher than $R$.Diagram (3) shows $R$ at a higher energy level and $P$ at a lower level, with the difference $\Delta H=74.8$.Diagram (4) correctly depicts the exothermic nature where $P$ is lower than $R$ and the enthalpy change is indicated.Checking the specific labels in the image, diagram (4) correctly places the product $P$ below reactant $R$ for an exothermic process.