Asecond messenger is:
a. a molecule that allows one cell to communicate with other cells
b. a chemical messenger released from synaptic terminals across synapses
c. a molecule that intiates a signaling cascade in response to hormone binding on a cell membrane receptor and activating a g protein.
d. a hormone that binds to a cell membrane hormone receptor and triggers activation of another messenger system.

c. a molecule that intiates a signaling cascade in response to hormone binding on a cell membrane receptor and activating a G protein.  

Explanation:

The second messenger is an intracellular signaling molecule that transmits signals within cells. These molecules are involved in the propagation of signals from the external to the intracellular medium.

These molecules promote initiation of intracellular signal transduction cascades. The second messenger needs to be active by some other molecule that is usually a hormone hormone. When activation occurs, a protein called Protein G changes the conformation of the second messenger by separating its alpha and beta-gamma subunits.

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i believe the answer would be b. hope it : )