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Secondary messengers are small molecules that mediate the signal transduction pathway by receiving the signals from cell surface receptors and transferring them to the effector proteins.

Generally, they are deficient in concentration when cells are not stimulated by a signal. After getting the stimulation, they diffuse rapidly from the source and bind the target proteins.

When epinephrine binds to the plasma membrane, it elevates the concentration of cyclic AMP in the cytoplasm. For this purpose, it binds to a G-protein coupled receptor that activates adenylyl cyclase, which catalyzes the synthesis of cyclic AMP.

Increased levels of cAMP induceactivation of a serine/threonine kinase called protein kinase A that phosphorylates other proteins.

Thus,cAMP works as a second messenger in the epinephrine transduction pathway.

Therefore, the given option is true.

G proteins are the proteins by which receptors present on the cell surface interact when epinephrine-like signals bind these receptors, so these are not the secondary messenger.

Thus, the G protein activates the receptors on the cell surface.

Therefore, the given option is false.

GTP is the molecule that activates the G protein by attaching with it, so it is not a part of the signal cascade.

Thus, GTP only activates the G-protein to work receptor protein in signal transduction.

Therefore, the given option is false.

Adenylyl cyclase is the enzyme present in the plasma membrane activated by receptor proteins and catalyzes the synthesis of cyclic AMP that relays the signal to the interior of the cell.

Thus, adenylyl cyclase does not work as a secondary messenger.

Therefore, the given option is false.