What substance lines the trachea and bronchi to trap dust and bacteria?

Mucus is the correct choice because it serves a critical function in the respiratory system by trapping dust, bacteria, and other particles that enter the airways. The lining of the trachea and bronchi contains specialized cells called goblet cells, which produce mucus. This sticky substance helps to filter and clean the air as it passes through the respiratory passages, preventing harmful particles from reaching the lungs. Additionally, tiny hair-like structures called cilia work in conjunction with the mucus; they beat in a coordinated manner to push the trapped debris upward toward the throat, where it can be swallowed or expelled.

Other substances like saliva, plasma, and serum do not fulfill this specific protective role in the respiratory tract. Saliva is primarily involved in digestion and oral health, while plasma and serum, components of blood, are not related to respiratory mucosal functions. Hence, mucus is essential for the respiratory system's defense mechanisms.