**Core Concept**
G protein-coupled receptors (GPCRs) that activate adenylate cyclase stimulate the production of cyclic adenosine monophosphate (cAMP). This is a critical signaling pathway in various physiological processes, including hormonal regulation and cellular metabolism.

**Why the Correct Answer is Right**
Adenylate cyclase is an enzyme that catalyzes the conversion of ATP to cAMP. When a GPCR is activated by its ligand, it triggers the exchange of GDP for GTP on the Gα subunit, which in turn activates adenylate cyclase. This enzyme then increases cAMP production, leading to the activation of protein kinase A (PKA) and subsequent downstream effects. The correct answer is an enzyme that stimulates adenylate cyclase activity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because phosphodiesterase is an enzyme that breaks down cAMP, not stimulates its production.
**Option B:** This option is incorrect because Gαi subunits inhibit adenylate cyclase activity, thus decreasing cAMP production.
**Option C:** This option is incorrect because Gαq subunits activate phospholipase C (PLC), which increases inositol trisphosphate (IP3) and diacylglycerol (DAG) production, but not cAMP.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that GPCRs can be coupled to different Gα subunits, leading to various downstream effects, including the stimulation or inhibition of adenylate cyclase activity.

**Correct Answer:** C.