Choice D is correct because cAMP binds to inactive PKA after being formed to activate PKA.
Choice A is incorrect because the enzyme adenyl cyclase (AC) acts on ATP to form cAMP. This has to take place first to form cAMP. Choices B and C are incorrect because these two reactions take place before cAMP is formed. A ligand such as epinephrine binds to the G protein-coupled receptor, resulting in guanosine triphosphate (GTP) binding to the G protein to activate it.
