Which ingredient is commonly associated with over-the-counter cold medications?

Pseudoephedrine is commonly associated with over-the-counter cold medications because it is a well-known decongestant. Its primary function is to relieve nasal or sinus congestion, making it popular in many cold and allergy remedies. Pseudoephedrine works by shrinking blood vessels in the nasal passages, leading to reduced swelling and congestion, thus allowing for easier breathing.

The other options, while relevant to medications, do not fit as commonly associated ingredients in the same context. Ibuprofen is an analgesic and anti-inflammatory but is typically used for pain relief and inflammation rather than specifically for cold symptoms. Codeine is an opioid and is generally used for its cough suppressant properties in prescription medications rather than over-the-counter options. Diphenhydramine is an antihistamine commonly found in allergy medications and can also be used for cold symptoms, particularly to relieve runny noses, but it is not as directly linked to decongestion as pseudoephedrine is. Hence, pseudoephedrine stands out as the primary ingredient associated with cold medications in the over-the-counter category.