What is a common sign indicating that a chimney needs cleaning?

A common sign indicating that a chimney needs cleaning encompasses all the options listed, demonstrating that multiple indicators can be present. Soot accumulation on the outside can suggest that there is a build-up inside the chimney as well, which can hinder proper ventilation. Slow burning fires may indicate that there is a blockage or residue in the flue, affecting the airflow and the efficiency of combustion. Unpleasant odors can also signal that creosote buildup is occurring, which may not only affect the smell but also present fire hazards. Since all these signs collectively point to the need for maintenance, option D effectively represents the comprehensive indicators that a chimney requires cleaning.