B. Potential energy as a function of nuclear separation.
As is given in the figure itself, it is potential energy curve for hydrogen
When two hydrogen atoms approach each other, the long range forces between the electronically charged electrons and nuclei (protons) come into play. Nuclei and electrons of hydrogen atoms repel each other as having same charge. On the other hand, nucleus of one atom is attracted to electron of the other. Thus lowering the potential energy of the system.
The electrons of approaching hydrogen atoms may have spin in the the same direction
The potential energy curve in the figure is for electrons having spin in the opposite directions.
Point (a) shows the hydrogen nuclei far apart and there is no interaction between them.
As the nuclei move closer the potential energy of the system starts decreasing. Point (b).
At a point (c) the inter-nuclei distance is optimum. This is
Any further decrease in the inter-nuclei distance increases the potential energy due to increasing inter-nuclear and inter-electronic repulsions.