Final answer:
The hydride ion (H-) in its ground state has two spin-paired electrons because it is composed of an extra electron in addition to the single electron a hydrogen atom naturally has, and these electrons occupy the same orbital with opposite spins.
Step-by-step explanation:
In its ground state, a hydride ion (H-) has two spin-paired electrons. This is because the hydride ion is an ion of hydrogen that has gained an extra electron, resulting in two electrons associated with it. According to the Pauli exclusion principle, these two electrons occupy the same orbital but must have opposite spins. Therefore, they are spin-paired.
The situation is analogous to the helium atom, which also has two electrons in its 1s orbital, and these electrons have opposite spins. Similarly, in the H2 molecule, the molecular orbital can hold two electrons with opposite spins as indicated in molecular orbital energy diagrams by one upward arrow and one downward arrow.
Therefore, the correct answer to the question is C. two spin-paired electrons.