Why is the tracheal wall supported by rings of cartilage?
to allow for the exchange of gases in the lungs
to trap remaining particles that have slipped through the upper respiratory tract
O to stop foreign objects from entering the airways
to prevent the airway from collapsing while air flows through it

The trachea is a structure that forms part of the respiratory system, between the larynx and the main bronchi, allowing air to pass into the lungs.

Structure of the trachea is formed by a series of incomplete cartilaginous rings that prevent the collapse of this structure during respiratory activity. The catílage is a firm and at the same time flexible tissue, which allows in the trachea the expansibility but avoids the closing of this important airway.

The other options are not correct because:

Allow the exchange of gases in the lungs is a function of the alveoli.
Trap remaining particles that have slipped through the upper respiratory tract is not a function of trachea.
Trachea is not capable of trapping foreign bodies.