Step 1 - Briefly remembering the definition of standard formation enthalpy
The standard formation enthalpy for a given substances is, by definition, the energy involved in producing that substance from its constituent elements.
In the case of an element, however, there's no way it can be produced from its constituent elements, as it is its own and only constituent element.
Let's take the formation of H as an example. Its standard formation equation would look like this:
Since there's no real transformation happening, there's no energy involved. So the enthalpy of formation in this case is 0.
Step 2 - Answering the exercise
The correct answer is therefore d) 0
Answer: d) 0