**Core Concept**
The axillary artery is a continuation of the subclavian artery, extending from the outer border of the first rib to the lower border of the teres major muscle. It is divided into three parts based on its relationship with the pectoralis minor muscle.

**Why the Correct Answer is Right**
The pectoralis minor muscle is a flat, triangular muscle that arises from the chest wall and inserts into the coracoid process of the scapula. As the axillary artery passes beneath the pectoralis minor muscle, it is divided into three parts: the first part extends from the outer border of the first rib to the lateral margin of the pectoralis minor muscle, the second part extends from the lateral margin of the pectoralis minor muscle to the lateral margin of the teres major muscle, and the third part extends from the lateral margin of the teres major muscle to the lower border of the teres major muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** The pectoralis major muscle does not divide the axillary artery into three parts. It is a larger, fan-shaped muscle that covers the pectoralis minor muscle and does not have a direct relationship with the axillary artery.
* **Option B:** The coracoid process of the scapula is the insertion site of the pectoralis minor muscle, but it does not divide the axillary artery. The coracoid process is a small, hook-like structure that provides attachment points for several muscles.
* **Option D:** The teres major muscle is a part of the axillary artery's boundary, but it does not divide the artery into three parts. The teres major muscle is a large, fan-shaped muscle that inserts into the humerus and has a relationship with the axillary artery, but it is not the structure that divides the artery.

**Clinical Pearl / High-Yield Fact**
When learning about the axillary artery, remember the three parts based on their relationship with the pectoralis minor muscle: first part (from the first rib to the pectoralis minor), second part (from the pectoralis minor to the teres major), and third part (from the teres major to the lower border of the teres major).

**Correct Answer: C. Pectoralis minor muscle.**