**Core Concept**
Cyclic AMP (cAMP) is a second messenger that mediates intracellular signaling in response to extracellular hormones or neurotransmitters. It functions by activating protein kinase A (PKA), which then phosphorylates target proteins to initiate cellular responses.

**Why the Correct Answer is Right**
cAMP does not directly activate adenylate cyclase (which produces cAMP) or cause Ca²⁺ release. Instead, cAMP binds to the regulatory subunit of protein kinase A (PKA), leading to the dissociation and activation of its catalytic subunit. This activated PKA phosphorylates downstream targets such as enzymes, ion channels, and transcription factors, thereby modulating cellular activity. This mechanism is central to processes like glycogen breakdown, neuronal excitability, and cardiac contractility.

**Why Each Wrong Option is Incorrect**
Option B: Adenylate cyclase is the enzyme that produces cAMP from ATP, not the target of cAMP action. cAMP acts *after* its synthesis.
Option C: Ca²⁺ release is mediated by intracellular stores (e.g., sarcoplasmic reticulum) via IP₃ or ryanodine receptors, not by cAMP. While cAMP can modulate Ca²⁺ channels indirectly, it does not directly cause Ca²⁺ release.
Option D: Since only A is correct, "all" is incorrect as it includes false statements.

**Clinical Pearl / High-Yield Fact**
Remember: cAMP → activates PKA → phosphorylates proteins. This pathway is crucial in hormone signaling (e.g., epinephrine in liver) and is a classic example of second messenger systems.

✓ Correct Answer: A. Activation of protein kinase