The standard molar enthalpy of formation ($\Delta _fH^{\circ}$) is defined as the enthalpy change for the formation of exactly one mole of a compound from its constituent elements in their most stable states of aggregation (reference states) . According to the sources, the reference state for carbon at 25°C and 1 bar is graphite [$C(graphite)$], and for hydrogen, it is dihydrogen gas [$H_2(g)$] . The standard state of methane at 298 K is gaseous [$CH_4(g)$] . Therefore, the equation $C(graphite) + 2H_2(g) \rightarrow CH_4(g)$ correctly represents the standard heat of formation . Other options are incorrect because they use an unstable allotrope (diamond), an incorrect physical state for the product (liquid methane), or non-molecular forms of the elements (atomic hydrogen) .