The correct answer is option 'D', the mouth.

The mouth plays a crucial role in the process of eating our food. It is the primary organ involved in the intake of food and the initial step in the digestion process. Let's explore the details of why the mouth is used for eating.

1. The Function of the Mouth:
The mouth is responsible for various functions related to eating. It is equipped with several important structures that aid in the process of digestion. Some of the main functions of the mouth include:
- Ingestion: The mouth receives and takes in the food.
- Mechanical Digestion: The teeth present in the mouth grind and break down the food into smaller pieces, making it easier to swallow and digest.
- Chemical Digestion: Saliva, produced by the salivary glands in the mouth, contains enzymes that start breaking down the food chemically, particularly starches.
- Sensory Perception: The taste buds on the tongue help us sense the flavor of the food, allowing us to enjoy our meals.

2. Structures Involved:
The mouth consists of several important structures that contribute to the eating process. These structures include:
- Lips: The lips help in guiding the food into the mouth and preventing it from falling out.
- Teeth: The teeth play a vital role in chewing and grinding the food into smaller pieces, making it easier to swallow and digest.
- Tongue: The tongue helps in moving the food around the mouth, mixing it with saliva, and pushing it toward the throat for swallowing.
- Salivary Glands: These glands produce saliva, which moistens the food, making it easier to swallow and initiating the process of chemical digestion.

3. The Eating Process:
When we eat, the food enters the mouth through the lips. The teeth then chew and grind the food, breaking it down into smaller pieces. The tongue mixes the food with saliva, forming a soft mass called a bolus. The bolus is then pushed toward the throat by the tongue's movements, initiating the swallowing process.

Once the food is swallowed, it travels through the esophagus and into the stomach, where further digestion takes place. The mouth, therefore, acts as the entry point for food into the digestive system, playing a crucial role in the initial stages of digestion.

In conclusion, the mouth is the body part we use to eat our food. It performs various functions such as ingestion, mechanical digestion, chemical digestion, and sensory perception. The structures present in the mouth, including the lips, teeth, tongue, and salivary glands, contribute to the process of eating and the initial stages of digestion.