Can anyone explain this to me?

The answer is D, the definition of standard enthalpy of formation is as follows: when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements. The last two words being especially important to this question. We would rule out A, B and C all because the product is formed from a mixture of an element and a compound and not just of elements. In other questions the reactants may all be in elemental form so you must look at the products being one mole or the reactants not being in their standard state.