Which of the following is an example of an expectorant?

An expectorant is a type of medication that helps to loosen and expel mucus from the airways, making it easier to clear congestion caused by respiratory conditions. Guaifenesin is the well-known example of an expectorant. It works by thinning the mucus in the airways, facilitating a productive cough, which is particularly beneficial for individuals suffering from conditions like bronchitis or other respiratory infections that produce excessive mucus.

Other medications listed, such as diphenhydramine and benzonatate, serve different purposes. Diphenhydramine is an antihistamine used primarily to relieve allergy symptoms and can actually dry up secretions, which is contrary to the action of an expectorant. Benzonatate is a cough suppressant that works by numbing the throat and lungs, thus inhibiting the cough reflex rather than promoting mucus clearance. Codeine, while also a cough suppressant, is an opioid and does not facilitate mucus expulsion.

Therefore, guaifenesin is the only option among those listed that effectively acts as an expectorant, making it the correct choice.