Answer:
A molecule containing readily 'detachable' phosphate group, an inorganic phosphate, and ADP.
Step-by-step explanation:
For substrate-level phosphorylation, take glycolysis as an example. Phosphoenolpyruvate has a readily 'detachable phosphate', which with the help of Pyruvate Kinase, is able to be 'attached' to ADP. Both Phosphoenolpyruvate and ADP act as substrates.
For oxidative phosphorylation, take ATP synthase as an example. Due to the proton motive force, the 'motor' of the enzyme allows the knob to spin, changing affinities for ADP, Pi (inorganic phosphate), and ATP. Thus, to produce ATP, ADP and Pi are required as substrates.
In all, a molecule containing readily 'detachable' phosphate group, an inorganic phosphate, and ADP can serve as substrates for ATP production.