Answer:
Phosphorylation of ADP, i.e. ATP synthesis
Step-by-step explanation:
Long answer: The protons from the inter-membrane space flow into the matrix of the mitochondrion through ATP synthase. As they are moving down the concentration gradient, energy is released and this is used by ATP synthase to phosphorylate ADP, i.e. add inorganic phosphate to ADP in order to create ATP.