The correct answer is option 'D': Esophagus

The esophagus is a long tube that runs from the mouth to the stomach. It is an important part of the digestive system as it plays a crucial role in transporting food from the mouth to the stomach for further digestion and absorption.

Here is a detailed explanation of the esophagus:

1. Anatomy of the Esophagus:
- The esophagus is a muscular tube that measures about 25 centimeters in length and 2 centimeters in diameter.
- It starts at the back of the throat (pharynx) and extends down through the chest, passing through the diaphragm, and joins the stomach.

2. Function of the Esophagus:
- The primary function of the esophagus is to transport food and liquids from the mouth to the stomach.
- It accomplishes this through a series of coordinated muscular contractions called peristalsis.
- Peristalsis helps propel the food bolus down the esophagus and into the stomach.

3. Structure of the Esophagus:
- The walls of the esophagus consist of several layers, including mucosa, submucosa, muscularis externa, and adventitia/serosa.
- The innermost layer, the mucosa, secretes mucus to lubricate the passage of food.
- The muscularis externa is made up of both smooth and skeletal muscles, which work together to propel the food.

4. Esophageal Disorders:
- Some common disorders of the esophagus include gastroesophageal reflux disease (GERD), esophagitis, esophageal strictures, and esophageal cancer.
- GERD occurs when the lower esophageal sphincter (LES) fails to close properly, allowing stomach acid to flow back into the esophagus.
- Esophagitis refers to inflammation of the esophagus, often caused by GERD or infection.
- Esophageal strictures are narrowed areas that can develop due to chronic inflammation or scarring.

In summary, the esophagus is a long tube that connects the mouth to the stomach and is responsible for transporting food and liquids for digestion. Understanding the anatomy and function of the esophagus helps us appreciate its vital role in the digestive process.