Final answer:
In the absence of a proton-motive force, experiments have shown that ATP can be c) both synthesized and hydrolyzed by ATP synthase.
Step-by-step explanation:
Experiments with H₂¹⁸O and ATP synthase in the absence of a proton-motive force showed that ATP can be both synthesized and hydrolyzed by ATP synthase.
ATP is synthesized when protons flow through ATP synthase, harnessing the electrochemical potential energy.
Conversely, ATP is hydrolyzed when it releases energy during the breakdown process.
So, the correct option is c) Both synthesized and hydrolyzed by ATP synthase.