Which substance is likely to be a cause of respiratory illnesses in developing countries? 

A)  asbestos 
B)  particulate matter 
C)  formaldehyde 
D)  cigarette smoke

Particulate matter, which can be derived from sources such as burning of solid fuels for cooking and heating, is a common cause of respiratory illnesses in developing countries due to limited access to clean energy sources and poor indoor air quality. Asbestos, formaldehyde, and cigarette smoke may also contribute to respiratory illnesses, but they are not as prevalent in developing countries.