why don't electrons, which are attracted to the protons in the nucleus, simply fall into the nucleus?

electrons are fermions, they can only access an energy level if it has not already been occupied by another electron. Thanks to this principle, there is a minimum energy level within the atom and it is not possible for an electron to collapse to the nucleus because it cannot exceed this} threshold.

Explanation:

I hope I have helped you.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.