**Core Concept**
The question tests the understanding of **referred pain** in lung cancer, specifically the involvement of the **mediastinal pleura** and its innervation by the **phrenic nerve**. The **phrenic nerve** arises from the cervical roots C3-C5, which also provide sensory innervation to the skin over the shoulder region.

**Why the Correct Answer is Right**
The tumor invading the **mediastinal pleura** stimulates the **phrenic nerve**, leading to **referred pain** in the shoulder region due to the common nerve roots. This is because the **phrenic nerve** shares the same cervical roots (C3-C5) that provide sensory innervation to the skin over the shoulder.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the tumor's location does not directly involve the **intercostal nerves**, which would be responsible for pain in the chest wall.
**Option B:** Incorrect as the **vagus nerve** is not primarily involved in transmitting pain from the **mediastinal pleura**.
**Option C:** Incorrect because the **recurrent laryngeal nerve** is not responsible for transmitting pain from the lung or **mediastinal pleura**.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to confuse the innervation of the **mediastinal pleura** with other structures, but remembering that the **phrenic nerve** is key to understanding **referred pain** patterns in lung cancer.

**Correct Answer:** D. Shoulder region.