**Core Concept:**
Heterotrimeric G proteins are a group of proteins involved in transducing signals from cell surface receptors to intracellular effectors, regulating various cellular processes such as cell growth, differentiation, and gene expression. They are composed of three subunits: Gα, Gβ, and Gγ.

**Why the Correct Answer is Right:**
The correct answer, D, is the α2-adrenergic receptor, which belongs to the family of G protein-coupled receptors (GPCRs). When activated by its ligand, norepinephrine, the α2-adrenergic receptor triggers the following sequence of events:

1. The receptor activates a Gαi/o subunit, which inhibits adenylyl cyclase, a key enzyme in the synthesis of cyclic AMP (cAMP).
2. Decreased cAMP levels lead to various downstream effects, such as inhibition of protein kinase A (PKA) and phosphodiesterase (PDE) activation, ultimately causing a reduction in cellular response.

**Why Each Wrong Option is Incorrect:**
A. The muscarinic M3 receptor is not coupled to a heterotrimeric G protein. Instead, it activates phospholipase C (PLC) and increases intracellular calcium levels.
B. The β-adrenergic receptor, like the α2-adrenergic receptor, is coupled to Gαs subunits that activate adenylyl cyclase and increase cAMP levels.
C. The dopamine D2 receptor is part of the D2-D4 receptor family and is coupled to Gαi/o subunits, similar to the α2-adrenergic receptor. However, the D2 receptor primarily inhibits adenylyl cyclase, leading to decreased cAMP production, which is different from the α2-adrenergic receptor's actions.

**Clinical Pearl:**
Understanding the coupling of different GPCRs to G protein subunits is crucial in understanding their effects on intracellular signaling pathways and cellular responses. This knowledge is essential for interpreting clinical scenarios involving medications targeting these receptors and their potential side effects.