The direct energy source that drives atp synthesis during respiratory oxidative phosphorylation is

b. the difference in H+ concentrations on opposite sides of the inner mitochondrial membrane.

Explanation:

As electrons move from NADH and FADH2 through the protein complexes of electron transport chain towards molecular oxygen, protons (H+) are transported from matrix to the intermembrane space.

This proton motive force with intermembrane space being more positive due to higher proton concentration with respect to the matrix side serves as an energy source for ATP synthesis. As protons are diffused back to the matrix side through Fo proton channels, the energy of the proton motive force is used to drive ATP synthesis.