The answer is b. droplets left on surfaces because scientific consensus, including guidance from organizations like the CDC and WHO, indicates that COVID-19 is primarily transmitted through respiratory droplets and aerosols that are inhaled directly from an infected person. While it's possible to get infected by touching contaminated surfaces (fomite transmission), this is considered a less common and less significant route compared to airborne and direct droplet transmission from breathing, talking, coughing, or sneezing.