The correct option is C S(rhombic)+O2(g)→SO2(g)+heat
Heat of formation is defined as the energy involved in the formation of 1 mole of that species from its constituents elements.
Since, in option A and C the products form is 1 mole only but the constituent elements taken are C(diamond) and H2. But, for carbon the chosen element is C(Graphite) as constituent element.
So, A and C are not the formation reaction
Hence, their heat of reaction is not equal to heat of formation.
In B, 2 moles of H2O is formed not 1.
Hence, it is also not formation reaction.
In option D, product formed is 1 mole and also the reactants chosen are constituent elements so its a formation reaction for SO2.
Hence, its heat of reaction is equal to heat of formation.
Hence, the answer is option D.