**Core Concept**
The fibrous pericardium is the tough, fibrous outer layer of the pericardium, which surrounds the heart. It is a dense layer of connective tissue that provides structural support to the heart.

**Why the Correct Answer is Right**
The phrenic nerve, specifically the ventral ramus of the cervical spinal nerves C3-C5, innervates the fibrous pericardium. This nerve provides sensory innervation to the fibrous pericardium, allowing it to detect any potential injury or irritation to the heart. The phrenic nerve is responsible for controlling the diaphragm, but it also has a role in the innervation of the fibrous pericardium.

**Why Each Wrong Option is Incorrect**
**Option A:** The pericardiophrenic nerve is a branch of the phrenic nerve, but it primarily supplies the parietal pleura and the pericardium's visceral layer, not the fibrous pericardium.
**Option C:** The T2 to T6 intercostal nerves are responsible for innervating the parietal pleura, but they do not supply the fibrous pericardium.
**Option D:** The vagus nerve is a major nerve that innervates various organs in the thorax, but it does not have a direct role in the innervation of the fibrous pericardium.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the fibrous pericardium is innervated by the phrenic nerve, which can be relevant in clinical scenarios where the heart is involved, such as cardiac tamponade or pericarditis.

**✓ Correct Answer: B. Phrenic Nerve**