Which of the following is a preventive measure against droplet spread diseases?

Covering the mouth when coughing or sneezing is a key preventive measure against droplet spread diseases. This action helps to contain respiratory droplets that can carry pathogens, such as viruses and bacteria, which are expelled when a person coughs, sneezes, or talks. By doing this, it reduces the risk of transmitting infectious agents to others in close proximity, thereby limiting the spread of diseases such as influenza, COVID-19, and other respiratory infections. This practice is particularly important in crowded or closed environments where people are in close contact.

Other options do not effectively address droplet transmission. For example, wearing long clothing outdoors has no direct impact on preventing the spread of respiratory diseases, as these diseases are not transmitted through skin contact or through clothing. Similarly, using bug spray is important for repelling insects and preventing vector-borne diseases like malaria or dengue fever, which are transmitted by mosquitoes, but it does not prevent droplet spread. Lastly, while staying indoors may reduce exposure to some infectious agents, it is not a practical nor a sustainable long-term preventive measure, as it can also lead to social isolation and other adverse health effects. Thus, the most effective preventive action from the choices provided is indeed covering one's mouth when coughing or sneezing.