Step 1: Think about how haze forms.
Haze appears when tiny particles in the air scatter and absorb sunlight before it reaches our eyes, blurring distant objects.
Step 2: Identify which pollutant does this best.
Fine particulate matter, especially PM2.5 and PM10, is close in size to the wavelength of visible light, which makes it very effective at scattering light.
Step 3: Compare with the other gases.
Ozone, carbon monoxide and nitrous oxide are gases that do not scatter visible light this way, so they are not the cause of haze.
\[ \boxed{\text{Fine particulate matter}} \]