Which of the following substances can serve as a cough suppressant?

Dextromethorphan is an effective cough suppressant commonly found in over-the-counter cough medications. It works by acting on the cough center in the brain, thereby reducing the cough reflex without affecting respiration. This makes it particularly useful for treating non-productive coughs where the aim is to alleviate the urge to cough rather than to expel mucus.

In contrast, guaifenesin is an expectorant, primarily used to help clear mucus from the airways rather than suppress a cough. Phenylephrine is a decongestant, typically used to relieve nasal congestion, and does not have direct effects on cough. Albuterol is a bronchodilator used to relieve asthma or COPD symptoms by relaxing the muscles in the airways, but it does not function as a cough suppressant. Thus, the role of dextromethorphan as a central nervous system depressant specifically targeting cough makes it the correct answer for this question regarding cough suppression.