UPSC Civil Services 2016 Prelims General Studies Paper I

(i) The API level is based on the level of 6 atmospheric pollutants, namely sulfur dioxide (SO2), nitrogen dioxide (NO2), suspended particulates smaller than 10µm in aerodynamic diameter (PM10), suspended particulates smaller than 2.5µm in aerodynamic diameter (PM2.5), carbon monoxide (CO), and ozone (O3) measured at the monitoring stations throughout. So by elimination we are left with option "b"