**Core Concept:** G-protein coupled receptors (GPCRs) are a large family of cell surface receptors that play a crucial role in transmitting signals across the cell membrane, regulating various physiological processes, and initiating intracellular signaling pathways. They are involved in the response to hormones, neurotransmitters, and other signaling molecules.

**Why the Correct Answer is Right:** G-protein coupled receptors are involved in the regulation of various physiological processes by binding to specific agonists (ligands) and activating intracellular signaling pathways. In the case of this question, we are discussing a G-protein coupled receptor that is activated by a specific ligand and leads to a specific response (e.g., adenylate cyclase activation, phospholipase C activation, or inhibition of certain ion channels).

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it does not specifically address a G-protein coupled receptor activated by a specific ligand.
B. This option is incorrect because it does not accurately describe the type of receptor activated by a specific ligand and leading to a specific response.
C. This option is incorrect as it does not represent a specific G-protein coupled receptor activated by a specific ligand and leading to a specific response.
D. This option is incorrect because it does not represent a specific G-protein coupled receptor activated by a specific ligand and leading to a specific response.

**Clinical Pearl / High-Yield Fact:** A key aspect of G-protein coupled receptors is their ability to couple with specific G proteins, which in turn activate downstream signaling pathways leading to a specific cellular response. This allows for precise regulation of cellular processes and ensures that the body can respond appropriately to various stimuli. Understanding G-protein coupled receptors is essential for understanding the pharmacology of many drugs and their mode of action.