**Core Concept**
The **Pancoast tumor** is a type of lung cancer located at the apex of the lung, often associated with specific histological subtypes. **Lung cancer** is a heterogeneous group of diseases, with various histological subtypes, including **squamous cell carcinoma**, **adenocarcinoma**, **small cell carcinoma**, and **large cell carcinoma**. The most common histological subtype associated with Pancoast tumor is related to the tumor's location and behavior.

**Why the Correct Answer is Right**
The most common histological subtype of lung cancer associated with **Pancoast tumor** is **squamous cell carcinoma**. This is because **squamous cell carcinoma** tends to arise from the proximal airways, including the bronchi and trachea, and can extend to the lung apex, leading to the characteristic presentation of a Pancoast tumor. The **squamous cell carcinoma** subtype is more commonly associated with this location due to its tendency to arise from the epithelial cells lining the airways.

**Why Each Wrong Option is Incorrect**
**Option A:** Not provided, assuming it's not the correct answer.
**Option B:** Not provided, assuming it's not the correct answer.
**Option C:** Not provided, assuming it's not the correct answer.
**Option D:** Not provided, assuming it's not the correct answer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that **Pancoast tumors** are often associated with **squamous cell carcinoma** and can present with unique clinical features, such as **Horner's syndrome** and **arm pain**, due to the tumor's location at the lung apex.

**Correct Answer:** D. Squamous cell carcinoma.