The correct answer is option 'D', incomplete cartilaginous rings. The trachea, also known as the windpipe, is a tube-like structure that connects the larynx (voice box) to the bronchi in the lungs. It plays a crucial role in the respiratory system by allowing the passage of air to and from the lungs.

The trachea is composed of a series of C-shaped rings made of cartilage. These cartilaginous rings provide structural support and prevent the trachea from collapsing during breathing. However, these rings are not complete circles, but rather incomplete rings with a gap at the back. This gap is bridged by the trachealis muscle, a band of smooth muscle tissue.

The function of the incomplete cartilaginous rings and the trachealis muscle are as follows:

1. Incomplete cartilaginous rings:
- The C-shaped rings are made of hyaline cartilage, a firm connective tissue.
- The open part of the rings faces the esophagus, allowing it to expand when swallowing food.
- The cartilage rings provide support and maintain the structure of the trachea, preventing it from collapsing and blocking the airway.
- The gap at the back of the rings is filled by the trachealis muscle.

2. Trachealis muscle:
- The trachealis muscle is a band of smooth muscle fibers that connect the ends of the cartilaginous rings.
- It spans the posterior (back) part of the trachea and completes the circle, forming a continuous tube.
- The trachealis muscle is elastic and contractile, allowing it to constrict or relax to adjust the diameter of the trachea.
- Contraction of the trachealis muscle narrows the trachea, while relaxation widens it.
- This flexibility in the trachea's diameter helps regulate airflow and aids in the expulsion of mucus or foreign particles.

In conclusion, the trachea is prevented from collapsing by a combination of incomplete cartilaginous rings and the trachealis muscle. These structures provide the necessary support and flexibility for proper airflow and protection of the airway.

The trachea is prevented from collapsing by:
Incomplete cartilaginous rings:
- The trachea is supported by a series of C-shaped cartilaginous rings.
- These rings are incomplete, meaning they do not encircle the entire trachea.
- The rings are open at the back, allowing flexibility and movement.
Function of the cartilaginous rings:
- The cartilaginous rings provide structural support to the trachea.
- They help to maintain the shape and patency of the tracheal lumen, preventing it from collapsing.
- The rings are rigid enough to keep the trachea open during inhalation and exhalation.
- The open ends of the rings allow the trachea to expand and contract as needed.
Alternative options:
- Bony rings and chitinous rings are not correct options as they do not exist in the human trachea.
- Complete cartilaginous rings are also not correct as they would restrict the flexibility and movement of the trachea.
In summary:
The trachea is prevented from collapsing by the presence of incomplete cartilaginous rings. These rings provide structural support while allowing flexibility and movement of the trachea.