Final answer:
The structure with connective tissue, seromucous glands, and C-shaped cartilage rings is the trachea.
Step-by-step explanation:
The structure that is composed of connective tissue with seromucous glands and is supported by 16-20 C-shaped cartilage rings to prevent its collapse is the trachea. The trachea, also known as the windpipe, extends from the larynx to the lungs and is crucial for maintaining an open airway for respiration. Formed by stacked, C-shaped pieces of hyaline cartilage, it includes a fibroelastic membrane and the trachealis muscle, which together allow for slight stretching and prevent the trachea from collapsing. The tracheal wall is lined by pseudostratified ciliated columnar epithelium containing goblet cells which produce mucus to trap debris and pathogens.