Answer: D. Epinephrine binds its transmembrane receptor, which activates a G protein. The activated G protein stimulates adenylate cyclase to produce cAMP inside the cell; cAMP acts as the second messenger that relays the signal to intracellular targets.
A: omits the G protein, incorrectly making the receptor activate adenylate cyclase directly.
B: gives the pathway correctly but misidentifies cAMP as the first messenger; epinephrine is the first messenger.
C: reverses the order of G protein and adenylate cyclase activation.
Remember: the first messenger binds the receptor; the second messenger relays the signal inside the cell.