Explanation:

Emissions from Petrol Vehicles
Emissions from petrol cars have been dramatically reduced by the introduction of catalytic converters, which oxidise pollutants such as CO to less harmful gases such as CO2. When compared to petrol cars without catalysts, catalyst cars have much lower CO, HC and NOx emissions, at the expense of CO2 emissions, which increase due to the oxidation of carbon monoxide to CO2.
Emissions from Diesel Vehicles
Diesel fuel contains more energy per litre than petrol and coupled with the fact that diesel engines are more efficient than petrol engines, diesel cars are more efficient to run. Diesel fuel contains no lead and emissions of the regulated pollutants (carbon monoxide, hydrocarbons and nitrogen oxides) are lower than those from petrol cars without a catalyst. 
LPG & CNG
On a cycle representing congested urban traffic, both LPG and CNG outperform petrol powered vehicles on emissions of carbon monoxide (CO). Indeed, emissions of CO from CNG powered vehicles are of the same order as those emitted by diesel vehicles.