**Core Concept**
The question is testing the student's knowledge of second messengers and their role in cellular signaling. cAMP (cyclic adenosine monophosphate) is a key second messenger involved in various cellular processes, including signal transduction and gene expression.

**Why the Correct Answer is Right**
cAMP is a second messenger for several receptors, including beta-adrenergic receptors, glucagon receptors, and certain G protein-coupled receptors (GPCRs). When these receptors are activated, they stimulate the production of cAMP through the activation of adenylyl cyclase, an enzyme that converts ATP into cAMP. The increase in cAMP levels triggers a cascade of downstream effects, including the activation of protein kinase A (PKA) and the regulation of gene expression.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because cAMP is a second messenger for beta-adrenergic receptors, which are a type of GPCR.
**Option B:** This option is incorrect because cAMP is a second messenger for glucagon receptors, which are also GPCRs.
**Option C:** This option is incorrect because cAMP is a second messenger for certain other GPCRs, such as vasopressin receptors.

**Clinical Pearl / High-Yield Fact**
cAMP plays a crucial role in various physiological processes, including glucose metabolism, cardiac function, and immune response. Understanding the role of cAMP in these processes is essential for diagnosing and treating diseases related to these systems.

**Correct Answer: D.**