Question

in the signal transduction process, the internal activator is known as the
a cell - surface receptor.
b g protein.
c ligand.
d second messenger.

Explanation:

In signal - transduction, cell - surface receptors bind to ligands. G proteins are involved in the initial steps of some signaling pathways. A ligand is a molecule that binds to a receptor. Second messengers are internal activators that relay signals from the cell - surface to the interior of the cell.

Answer:

d. second messenger