**Core Concept**
The question is testing the student's knowledge of the anatomical differences between the right and left lungs. Specifically, it's asking about a feature that is unique to the left lung, which is a critical detail in chest radiology and thoracic surgery.

**Why the Correct Answer is Right**
The correct feature found only in the left lung is the **ligamentum arteriosum**. This ligament is a fibrous remnant of the ductus arteriosus, a vital fetal structure that allows blood to bypass the lungs and go directly to the systemic circulation. After birth, the ductus arteriosus closes and becomes the ligamentum arteriosum. The left lung has a right-sided ligamentum arteriosum, which is a distinguishing feature from the right lung. This is due to the unique anatomy of the fetal circulation and the positioning of the ductus arteriosus.

**Why Each Wrong Option is Incorrect**
* **Option A:** Not applicable, as the question is asking for a feature found only in the left lung, and option A is not specified.
* **Option B:** This is incorrect because the **hilum** is the point of entry and exit for structures such as the bronchus, pulmonary artery, and veins. While the hilum is different between the left and right lungs, it is not a unique feature of the left lung.
* **Option C:** This is incorrect because the **cardiac notch** is a feature of the left lung's upper lobe, but it is not unique to the left lung.
* **Option D:** This is incorrect because the **oblique fissure** is a feature of both lungs, dividing the upper and lower lobes.

**Clinical Pearl / High-Yield Fact**
Remember that the ligamentum arteriosum is a key landmark in thoracic surgery, particularly when performing procedures such as pulmonary lobectomies or pericardiectomy.

**Correct Answer: C. Cardiac notch.**