Asthma is a common long-term inflammatory disease of the airways of the lungs. Symptoms include episodes of wheezing, coughing, chest tightness, and shortness of breath. Environmental factors which are responsible for asthma include exposure to air pollution and allergens. The increased asthmatic attacks in certain seasons are due to the inhalation of seasonal pollen. Seasonal pollen is obtained by the dehiscence of pollen grain in a particular season to which a person may be allergic.So, the correct answer is 'Inhalation of seasonal pollen'.