Hey there!
Your answer is C, adding a phosphate group to ADP.
The chemical reaction would be ADP + Pi -----> ATP, where ATP synthase is the enzyme that catalyzes the reaction.
Using "synthase" in the name of a protein means that the enzyme is used to synthesize a molecule. For example, "citrate synthase" synthesizes the citrate molecule. Therefore, ATP synthase synthesizes ATP.
Hope this helps!
