A smoker often coughs primarily because toxins irritate the lining of the respiratory tract. The inhalation of smoke introduces various harmful substances that irritate the airways, leading to inflammation and increased mucus production. This irritation triggers the cough reflex as the body attempts to clear the airways.
Additionally, while tar sticks to the cilia, making them less effective at clearing mucus and debris, and mucus gets thicker, both of which can contribute to coughing, the initial cause for the cough relates to the irritation from toxins.
Bronchi restricting airflow can also contribute to respiratory issues in smokers, but it is typically a result of long-term damage rather than the direct cause of the cough itself.
In summary, while all the options can be related to a smoker's condition, the most direct reason for coughing is the irritation caused by toxins.